fix: replace broken test_compile_code and add compile failure test

[sakshamg19]

Fixes #161

Description

test_compile_code in gdbui_server/flask_test.py was mocking self.client.post which is the Flask test client itself. This meant
the actual /compile route handler never ran during the test. The test was only checking that a mock returned what it was set up to return, so it would always pass regardless of what the real code does.

Changes

Fixed by mocking subprocess.run instead and sending a real request through Flask, which now exercises JSON parsing, file writing, and response building.

Added test_compile_code_failure to cover the g++ non-zero exit code path (lines 88-89 in main.py), which had zero test coverage.

Added *.cpp and *.exe to gdbui_server/.gitignore to prevent compiled test artifacts from being accidentally committed.

Testing

python3 -m unittest flask_test.TestGDBRoutes.test_compile_code -v passes
python3 -m unittest flask_test.TestGDBRoutes.test_compile_code_failure -v passes
Full test suite (python3 -m unittest flask_test -v) passes with 21/21 tests