fix(ledger): guard custom currency charge payments

[turip]

Summary

• reject custom-currency payment authorization in the flat-fee and usage-based ledger adapters
• reject custom-currency payment settlement through the same explicit unsupported-currency error

Behavioral change

Custom-currency overage accrual is not yet supported by the ledger adapters, but the payment handlers did not enforce the same boundary. If reached independently, they could attempt to resolve a custom-currency receivable using a fiat payment amount.

The payment handlers now return ErrCustomCurrencyNotSupported before resolving or committing a ledger transaction. Fiat charge payment behavior is unchanged. These guards are intentionally temporary and should be removed when OM-435 implements fiat settlement for custom-currency overage.

Validation

go test ./openmeter/ledger/chargeadapter

Ticket: OM-435

Summary by CodeRabbit

• Bug Fixes
  • Added clear validation when authorizing and settling flat-fee payments with custom currencies.
  • Added the same validation for usage-based payments.
  • Custom-currency transactions are now rejected early with a consistent, actionable error instead of proceeding through payment processing.

Greptile Summary

This PR prevents unsupported custom-currency payment transactions from reaching ledger resolution or commit.

• Adds early custom-currency rejection to flat-fee payment authorization and settlement.
• Adds equivalent guards to usage-based payment authorization and settlement.
• Preserves existing fiat payment behavior and wraps the shared unsupported-currency sentinel for error inspection.

Confidence Score: 5/5

The PR appears safe to merge, with unsupported custom-currency payments rejected before ledger mutations while fiat behavior remains unchanged.

The four new checks return the shared unsupported-currency error before transaction resolution or commit, and current workflow ordering does not expose a conflicting reachable no-op path.

Important Files Changed

Filename	Overview
openmeter/ledger/chargeadapter/flatfee.go	Adds correctly positioned custom-currency guards before flat-fee payment transaction resolution and ledger commits.
openmeter/ledger/chargeadapter/usagebased.go	Adds matching custom-currency guards to usage-based authorization and settlement; no currently reachable regression was identified.

_{Reviews (1): Last reviewed commit: "feat: guard custom currency payments" | Re-trigger Greptile}

Context used:

• Knowledge Base — Credit Grants, Balances, and Entitlements

[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 Plus

Run ID: 30bb0d97-d084-4347-ba51-d4a3f769ba85

📥 Commits

Reviewing files that changed from the base of the PR and between 887e0ca and a5636e4.

📒 Files selected for processing (2)

• openmeter/ledger/chargeadapter/flatfee.go
• openmeter/ledger/chargeadapter/usagebased.go

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📝 Walkthrough

Walkthrough

Flat-fee and usage-based charge adapters now reject custom currencies in payment authorization and settlement handlers. Each handler returns a wrapped meta.ErrCustomCurrencyNotSupported error before processing continues.

Changes

Custom currency validation

Layer / File(s)	Summary
Add currency guards openmeter/ledger/chargeadapter/flatfee.go, openmeter/ledger/chargeadapter/usagebased.go	Authorization and settlement handlers reject custom currencies before validation or transaction processing. They return an error wrapping meta.ErrCustomCurrencyNotSupported.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• openmeterio/openmeter#4738: Introduces broader custom-currency support that these handlers explicitly reject.
• openmeterio/openmeter#4780: Modifies custom-currency handling in the same usage-based payment handlers.
• openmeterio/openmeter#4840: Uses meta.ErrCustomCurrencyNotSupported for custom-currency rejection at another billing boundary.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the addition of guards that reject custom-currency charge payments in the ledger adapters.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/om-435-custom-currency-payment-guards

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