test(bff): add unit tests to meet CI coverage threshold

[golkity]

Description

This PR introduces the foundational test suite for the BFF service, raising the code coverage to meet the 80% threshold required by our CI pipeline.

What was tested

• API Handlers: Added tests for the primary HTTP routes (e.g., /auth, /stream) using Axum's Router and tower::ServiceExt (oneshot requests) without spinning up a real network server.
• Error Handling: Verified that AppError correctly maps to the expected HTTP status codes (e.g., 401 Unauthorized for InvalidCredentials).
• Middleware (if applicable): Covered JWT validation and rate-limiting logic.

Note

The tests run entirely in memory and do not require active gRPC downstream services to pass.

Tip

Reviewers can run cargo llvm-cov test --all-features locally to see the exact lines covered by this PR.

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by finnik