fix: add max_length to code_context_api.py BuildContextRequest.repoPath

[norascott407]

Hi there, first time contributing here. I was reading through the code_context_api module and noticed that the BuildContextRequest model does not constrain the length of the repoPath field. Since this path is used in filesystem operations, an unbounded string could be problematic.

This adds a max_length=1024 constraint to the repoPath Field in BuildContextRequest (backend/app/modules/code/code_context_api.py, line 32). Pydantic handles validation automatically and returns a 422 error with a clear message when the limit is exceeded.

I also added a test file (backend/tests/test_pr_16_code_context_validation.py) with four cases:

• Normal path is accepted
• Path at exactly 1024 characters is accepted
• Path at 1025 characters is rejected with ValidationError
• Path at 50,000 characters is rejected

All tests pass. Let me know if anything needs adjusting.