test: add unit coverage for inferQuotaPoolKey

[ashstru9-cloud]

Summary

• inferQuotaPoolKey in server/src/services/provider-quota.ts had no unit tests, despite being a pure function that maps platforms/models to quota pool keys.
• Adds tests covering the openrouter free/account split, several fixed shared-pool platforms, and the fallback behavior for unrecognized platforms (with and without a model id).

Test plan

• npx vitest run src/__tests__/services/provider-quota.test.ts passes (4/4)