refactor: rename flat fee credit allocation handler

[turip]

Summary

• rename the flat-fee credit allocation handler to OnAllocateCredits
• route credit-only flat-fee allocation through the same handler
• rename flat-fee credit allocation correction handler to OnCorrectCreditAllocations

Verification

• direnv exec . env POSTGRES_HOST=127.0.0.1 GOCACHE=/tmp/go-build-openmeter-handler-renames go test -count=1 -tags=dynamic ./openmeter/billing/charges/flatfee/... ./openmeter/billing/charges/service ./openmeter/ledger/chargeadapter ./test/credits
• direnv exec . env GOCACHE=/tmp/go-build-openmeter-handler-renames make lint-go-fast

Summary by CodeRabbit

• Refactor

  • Consolidated credit allocation and correction flows into unified allocation events, simplifying billing behavior during invoice and payment processing.
  • Standardized allocation input handling and validation to prevent negative pre-tax allocations.

• Tests

  • Updated and reorganized test coverage to exercise the new allocation and correction flows end-to-end.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 268dcd4 and afc4fca.

📒 Files selected for processing (1)

• openmeter/billing/charges/testutils/handlers.go

🚧 Files skipped from review as they are similar to previous changes (1)

• openmeter/billing/charges/testutils/handlers.go

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📝 Walkthrough

Walkthrough

This PR consolidates flat-fee credit allocation and correction handling by replacing three handler methods with two unified ones, introducing OnAllocateCreditsInput (PreTaxAmountToAllocate) and CorrectCreditAllocationsInput (AllocateAt), and updating services, ledger adapter, mocks, and tests to the new contract.

Changes

Flat-Fee Handler Consolidation

Layer / File(s)	Summary
Handler contract and input types openmeter/billing/charges/flatfee/handler.go	New OnAllocateCreditsInput validates non-negative PreTaxAmountToAllocate; CorrectCreditAllocationsInput adds AllocateAt. Handler interface replaces OnAssignedToInvoice, OnCreditsOnlyUsageAccrued, and OnCreditsOnlyUsageAccruedCorrection with OnAllocateCredits and OnCorrectCreditAllocations.
Service layer handler invocations openmeter/billing/charges/flatfee/service/realizations/creditsonly.go, .../credittheninvoice.go, .../realizations/correct.go	Service code now calls OnAllocateCredits for allocations and OnCorrectCreditAllocations for corrections; input validation and error wrapping updated, downstream realization/lineage wiring unchanged.
Ledger adapter handler implementation openmeter/ledger/chargeadapter/flatfee.go	flatFeeHandler implements OnAllocateCredits (zero short-circuit, validation, accepts CreditThenInvoice/CreditOnly) and OnCorrectCreditAllocations; collector uses PreTaxAmountToAllocate.
Test mock implementations openmeter/billing/charges/testutils/handlers.go, openmeter/billing/charges/service/handlers_test.go	Mocks and test handler doubles implement OnAllocateCredits and OnCorrectCreditAllocations with nil-callback guards; return allocation/correction inputs using ServicePeriod and PreTaxAmountToAllocate.
Ledger adapter tests openmeter/ledger/chargeadapter/flatfee_test.go	Tests reorganized around TestOnAllocateCredits and TestOnCorrectCreditAllocations; helpers updated to construct OnAllocateCreditsInput (PreTaxAmountToAllocate).
Service-level invoicable tests openmeter/billing/charges/service/invoicable_test.go	Multiple flat-fee tests wired to onAllocateCredits and updated assertions to validate PreTaxAmountToAllocate and callback invocation counts across scenarios.
Additional service tests and integration openmeter/billing/charges/service/lineage_test.go, taxcode_test.go, test/credits/sanity_test.go	Lineage and tax-code tests switched to onAllocateCredits; sanity comments updated to reflect the new allocation hook.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

• openmeterio/openmeter#4009: Overlaps prior credits-only lifecycle work that this PR consolidates into OnAllocateCredits.
• openmeterio/openmeter#4287: Touches tax-code tests and handler callback wiring around flat-fee allocation flows.
• openmeterio/openmeter#4035: Related changes to correction orchestration and credits-only flows that intersect with OnCorrectCreditAllocations.

Suggested labels

release-note/misc

Suggested reviewers

• tothandras
• GAlexIHU

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately describes the main change: renaming flat-fee credit allocation handlers from generic/assignment-based names (OnAssignedToInvoice, OnCreditsOnlyUsageAccrued) to more semantically precise ones (OnAllocateCredits, OnCorrectCreditAllocations).
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/flatfee-handler-renames

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@openmeter/billing/charges/testutils/handlers.go`:
- Around line 34-41: The mockFlatFeeHandler.OnAllocateCredits currently always
returns one allocation entry; change it to mirror the production handler by
returning nil, nil when input.PreTaxAmountToAllocate is zero. Update the
mockFlatFeeHandler.OnAllocateCredits implementation to check
input.PreTaxAmountToAllocate and only build and return
creditrealization.CreateAllocationInputs when the amount is non-zero, otherwise
return nil, nil; reference the mockFlatFeeHandler.OnAllocateCredits function and
the input.PreTaxAmountToAllocate field when making the change.

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Fix all unresolved CodeRabbit comments on this PR:

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 4af9315e-d4af-410a-a59e-196b18625c44

📥 Commits

Reviewing files that changed from the base of the PR and between 6161e62 and 268dcd4.

📒 Files selected for processing (12)

• openmeter/billing/charges/flatfee/handler.go
• openmeter/billing/charges/flatfee/service/realizations/correct.go
• openmeter/billing/charges/flatfee/service/realizations/creditsonly.go
• openmeter/billing/charges/flatfee/service/realizations/credittheninvoice.go
• openmeter/billing/charges/service/handlers_test.go
• openmeter/billing/charges/service/invoicable_test.go
• openmeter/billing/charges/service/lineage_test.go
• openmeter/billing/charges/service/taxcode_test.go
• openmeter/billing/charges/testutils/handlers.go
• openmeter/ledger/chargeadapter/flatfee.go
• openmeter/ledger/chargeadapter/flatfee_test.go
• test/credits/sanity_test.go