feat: add OpenAPI examples for analysis endpoints

[Neelr1912]

Closes #205

What changed

• Added openapi_examples to Body(...) on all four analysis endpoints:
  • POST /explanation/ — 3 examples (Python Fibonacci, JS async fetch, auto-detect Java)
  • POST /debugging/ — 3 examples (Python division-by-zero, JS undefined var, clean code)
  • POST /suggestions/ — 3 examples (Python missing types, JS var usage, high-score baseline)
  • POST /analyze/ — 3 examples (Python bubble sort, JS weak password, Python SQL injection)
• Added model_config with json_schema_extra to CodeRequest in schemas.py
  for the Schema tab in Swagger UI

How to verify

1. Run uvicorn app.main:app --reload from the backend/ directory
2. Open http://127.0.0.1:8000/docs
3. Expand any of the four endpoints → click the Examples dropdown in the request body
4. Select a sample → hit Execute

Checklist

• pytest -v passes
• No new dependencies added
• No secrets committed
• Branch is up-to-date with main

[Neelr1912]

@imDarshanGK Hi! I've added OpenAPI examples for all four analysis endpoints as requested in #205. All tests pass locally. Ready for your review