fix(billing): detect FK violations from error messages

[g1331]

Summary

Fix billing snapshot FK retry detection when the deployed Postgres/Drizzle error only exposes the violated constraint in the error message text.

Related Issue

Follow-up to deploy smoke failure observed in run 26326869522.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (code improvement without changing functionality)
• Tests (adding or updating tests)
• Build/CI (changes to build system or CI configuration)

Changes

• Parse violates foreign key constraint "..." from top-level and cause error messages when Postgres FK metadata fields are unavailable.
• Keep the existing exact FK column mapping so only known api_key_id and upstream_id billing snapshot constraints are retried with NULL.
• Add a regression test for the production-shaped wrapped error that only exposes the upstream FK constraint in message text.

Test Plan

• Local tests pass (pnpm test:run tests/unit/services/billing-cost-service.test.ts)
• Type check passes (pnpm exec tsc --noEmit)
• Lint passes (eslint via commit hook)
• Manual testing completed

Checklist

• Code follows the project's coding standards
• Tests have been added where necessary
• Documentation has been updated (if applicable)
• Changes do not introduce security vulnerabilities
• Commit messages follow conventions

Screenshots

N/A

Additional Notes

The previous fix handled constraint_name and constraint fields, but the deployed error in run 26326869522 did not expose those fields to the application-level detector. The constraint name was still present in the error message text, so this patch uses that text as the final FK detection source.

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.66%. Comparing base (67c5607) to head (03a03f9).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #173   +/-   ##
=======================================
  Coverage   78.65%   78.66%           
=======================================
  Files         145      145           
  Lines       11521    11538   +17     
  Branches     3985     3992    +7     
=======================================
+ Hits         9062     9076   +14     
  Misses       1631     1631           
- Partials      828      831    +3     

Flag	Coverage Δ		*Carryforward flag
typescript	76.02% <ø> (ø)		Carriedforward from 67c5607
verify	74.19% <82.35%> (+<0.01%)	⬆️

*This pull request uses carry forward flags. Click here to find out more.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.