test: improve history endpoint edge case coverage

[jyotish6699]

Summary

This PR improves test coverage for the history module by adding validation, boundary, and database behavior tests.

Added Tests

• Verify oversized code submissions are rejected (max_length=50000)
• Verify pagination limit upper-bound validation
• Verify pagination limit lower-bound validation
• Verify search limit upper-bound validation
• Verify code preview truncation behavior
• Verify SHA256 code hash generation
• Verify deleting the same history entry twice returns a 404 response

Testing

Executed successfully:

pytest backend/tests/test_history.py -v
pytest

Results:

• 15/15 history tests passing
• Full test suite passing (423 tests)

Impact

• Improves confidence in API validation behavior
• Covers important database-specific functionality
• Helps prevent regressions in history-related features
• Strengthens edge-case coverage

Closes #833