fix: reject duplicate code-review submission ids with 409 (issue #1871)

[ionfwsrijan]

Problem

POST /api/code-review/submit silently overwrites an existing submission when the caller re-uses a submission_id. submit_code does an unconditional self.submissions[submission_id] = submission, destroying any existing submission — including its review_count and metrics — while reviews already started against the old submission keep pointing at the stale ID. There was no error path, so clients could not detect the collision.

Fix

• Added SubmissionAlreadyExistsError in src/utils/code_review.py.
• submit_code now raises SubmissionAlreadyExistsError when submission_id already exists, instead of clobbering the record.
• The route catches the new exception and returns HTTP 409 Conflict with a clear message, keeping the existing record (review progress, metrics) intact.

Files changed

• src/utils/code_review.py — new SubmissionAlreadyExistsError; existence check in submit_code.
• src/routes/main_routes.py — catch SubmissionAlreadyExistsError and return 409.
• tests/test_code_review.py — regression tests: duplicate submission_id is rejected; rejected re-submit leaves review_count/metrics/code untouched.

Testing

• Isolated test run of tests/test_code_review.py — 21 passed (19 existing + 2 new).
• Route simulation confirmed first submit returns 201, duplicate submit returns 409 with {"error": "Submission s1 already exists"}.

Closes #1871

[vercel]

@ionfwsrijan is attempting to deploy a commit to the komalsony234-1530's projects Team on Vercel.

A member of the Team first needs to authorize it.