[BUG] Missing Negative Test Coverage for Backend APIs #65

Bug Description

What is the problem?

Most backend tests currently check only the successful (happy-path) cases.
In real usage, external services like GitHub API, Gemini AI, and FAISS can fail, timeout, or return unexpected data.

These failure scenarios are currently not covered by tests.

Steps to Reproduce

1. Call backend APIs with an invalid or expired GitHub token
2. Simulate an empty or malformed response from GitHub or Gemini
3. Trigger a network timeout or service failure
4. Observe that failures are not properly handled or tested

Expected Behavior

The backend should:

• Handle failures gracefully
• Return clear and safe error responses
• Have test cases that validate this behavior

Actual Behavior

Failure scenarios are not tested, which may lead to:

• Silent crashes
• Unclear error responses
• Bugs appearing only in production

Component

Backend

Environment

Backend (FastAPI, AI services, FAISS)
Backend service (environment-independent)

Additional Context

This issue focuses only on adding test coverage.
No API behavior or features will be changed.
I’d like to work on this issue as part of SWoC — please assign it to me if approved.