test: add integration tests for one-shot token LD_PRELOAD library (#608)

* Initial plan

* test: add integration tests for one-shot token LD_PRELOAD library

Add comprehensive integration tests that verify the one-shot token protection
mechanism works correctly in both container and chroot modes. Tests verify:

- Protected tokens (GITHUB_TOKEN, COPILOT_GITHUB_TOKEN, OPENAI_API_KEY) can be read once
- Subsequent reads return empty/null (token has been cleared)
- Non-sensitive environment variables are not affected
- Multiple tokens are handled independently
- Behavior works with both shell (printenv) and programmatic (Python getenv) access
- Edge cases (empty values, nonexistent tokens, special characters) are handled

Tests address feedback from PR #604 review requesting integration tests for
the one-shot token feature to prevent regressions.

Co-authored-by: lpcox <[email protected]>

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: lpcox <[email protected]>

Author: Copilot