chore: add credits api extensions

[turip]

Overview

Add credits related api data.

Notes for reviewer

Summary by CodeRabbit

Release Notes

• New Features
  • Invoices now display credit allocations, showing which credits were applied to individual line items with their amounts and descriptions.
  • Invoice totals now include a dedicated credits total amount alongside existing discount and charge totals.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 4f019292-0e2a-45ae-98a6-948ec983219a

📥 Commits

Reviewing files that changed from the base of the PR and between 77577d3 and bf337e4.

⛔ Files ignored due to path filters (6)

• api/client/go/client.gen.go is excluded by !api/client/**
• api/client/javascript/src/client/schemas.ts is excluded by !api/client/**
• api/client/python/openmeter/_generated/models/__init__.py is excluded by !**/_generated/**, !api/client/**
• api/client/python/openmeter/_generated/models/_models.py is excluded by !**/_generated/**, !api/client/**
• api/openapi.cloud.yaml is excluded by !**/openapi.cloud.yaml
• api/openapi.yaml is excluded by !**/openapi.yaml

📒 Files selected for processing (7)

• api/api.gen.go
• api/spec/packages/legacy/src/billing/invoices/credits.tsp
• api/spec/packages/legacy/src/billing/invoices/invoice.tsp
• api/spec/packages/legacy/src/billing/invoices/main.tsp
• openmeter/billing/httpdriver/invoice.go
• openmeter/billing/httpdriver/invoice_test.go
• openmeter/billing/httpdriver/invoiceline.go

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📝 Walkthrough

Walkthrough

This PR adds end-to-end support for tracking credit allocations on invoices. It introduces a new InvoiceLineCreditAllocation TypeSpec model, extends invoice line and totals schemas with credit fields, implements backend mapping logic to convert internal credit data to API responses, and includes unit tests validating the credit serialization behavior.

Changes

Invoice Credit Allocations

Layer / File(s)	Summary
TypeSpec Credit Model api/spec/packages/legacy/src/billing/invoices/credits.tsp, api/spec/packages/legacy/src/billing/invoices/main.tsp	New InvoiceLineCreditAllocation model with read-only amount: Money and optional description fields; imported into invoice package.
TypeSpec Schema Extensions api/spec/packages/legacy/src/billing/invoices/invoice.tsp	InvoiceLineBase gains optional creditAllocations array; InvoiceTotals gains creditsTotal numeric field, both read-only.
Backend Credit Mapping openmeter/billing/httpdriver/invoiceline.go, openmeter/billing/httpdriver/invoice.go	New mapInvoiceLineCreditsToAPI helper converts internal billing.CreditsApplied to API CreditAllocations; integrated into detailed and usage-based line mappers; totals mapper includes CreditsTotal.
Unit Tests openmeter/billing/httpdriver/invoice_test.go	Three new tests verify mapTotalsToAPI includes credits total, mapInvoiceLineCreditsToAPI correctly maps credits with amount/description, and empty credits are omitted.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested labels

area/api, area/billing, release-note/misc

Suggested reviewers

• tothandras

🚥 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 title accurately reflects the main change: adding credit-related API extensions across TypeSpec models and Go HTTP driver mapping logic.
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

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/credits-api

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