fix(ledger): prioritize free grants in collection

[GAlexIHU]

Summary

• prioritizes nil/missing-cost-basis and explicit zero-cost-basis customer FBO sources before paid sources when credit priority ties
• stamps collection source entries with a priority version annotation for future ordering changes
• adds focused collector coverage and credits sanity coverage for the new burn-down order

Notes

• current promotional credit-purchase settlement still maps through the charge adapter as explicit zero cost basis; the collector rule now also handles nil/missing cost basis as free
• existing correction safety comes from persisted collection source order; old entries do not need migration
• legacy credit engine grant burn-down history cannot express this cost-basis tie-break because its grant model has no cost-basis field

Validation

• nix develop --impure .#ci -c env POSTGRES_HOST=127.0.0.1 go vet -tags=dynamic ./openmeter/ledger/collector ./test/credits
• nix develop --impure .#ci -c env POSTGRES_HOST=127.0.0.1 go test -tags=dynamic -run 'TestCollectCustomerFBOUsesNilCostBasisBeforePaidTieBreaker|TestCollectCustomerFBOUsesPriorityOrder|TestCollectCustomerFBOReleasesBreakageInExpiryOrder' ./openmeter/ledger/collector
• nix develop --impure .#ci -c env POSTGRES_HOST=127.0.0.1 go test -tags=dynamic -run 'TestSanitySuite/TestFlatFeeCreditThenInvoiceUsesFreeGrantBeforePaidPriorityTie' ./test/credits

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: d5b5f287-cc13-42d6-8035-a62bcd02f362

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/free-grant-burndown-priority

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[greptile-apps]

Greptile Summary

This PR updates FBO credit collection ordering for tied priorities. The main changes are:

• Treats free and missing cost basis sources ahead of paid sources.
• Adds a collection priority version annotation to new source entries.
• Adds focused collector and credits tests for the new burn-down order.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
openmeter/ledger/collector/collection_fbo.go	Adds free-cost-basis classification to both FBO source construction paths.
openmeter/ledger/collector/types.go	Orders free or missing cost basis sources before paid sources after credit priority ties.
openmeter/ledger/annotations.go	Adds the collection priority version annotation key.
openmeter/ledger/collector/collection_fbo_test.go	Adds collector coverage for nil-cost-basis credits at tied priority.
test/credits/sanity_test.go	Adds credits sanity coverage for free-before-paid tied priority collection.

_{Reviews (2): Last reviewed commit: "fix(ledger): treat missing cost basis as..." | Re-trigger Greptile}