fix: add timeout for code runner

[pavsoss]

Summary

This PR fixes the Code Runner plugin hanging on long-running Python snippets by enforcing execution time limits and adding regression tests to prevent future failures.

While implementing the timeout fix, the Code Runner execution command was also updated to use the current Python interpreter (sys.executable) instead of a hardcoded python3 command, improving compatibility across Windows, macOS, Linux, and virtual environments.

Changes Made

Backend Updates

plugins.py

• Replaced the hardcoded python3 execution command with sys.executable.
• Added/updated timeout handling for long-running code execution.
• Ensured timeout failures return a controlled error response instead of leaving the request hanging.

Tests

test_api.py

• Added test_coderunner_success to verify that normal Python snippets execute successfully and return expected output.
• Added test_coderunner_timeout to verify that infinite loops or long-running snippets are terminated after the configured timeout period.

Before

• Code Runner could hang indefinitely when executing snippets such as:

while True:
    pass

After

• Long-running snippets are automatically terminated after the timeout duration with a clear error response.
• Code execution works consistently by using the active Python interpreter.
• Normal snippets continue to execute successfully.

Testing

Verified locally by running the test suite:

pytest -v

All tests passed successfully, including the new Code Runner execution and timeout regression tests.

Result

The Code Runner is now more reliable, cross-platform compatible, and protected against indefinitely running snippets.

Closes #237

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