chore(plugin): clean up imports and expected test exceptions - #1665

Summary

Remove unused imports and clarify Python imports and the expected exception in an archive concurrency test, preserving production behavior.

Changes

• Remove unused schema and error-helper imports from the MCP app.
• Import ctypes.wintypes and unittest.mock consistently while retaining their existing local names.
• Catch the expected SystemExit in the archive registration test instead of every BaseException.

Testing

Passed:

• Ruff 0.16.10 check and format check over plugins/codex-security using its pyproject.toml.
• pnpm --dir sdk/typescript run build:ci.
• node .github/scripts/check_plugin_source_compatibility.mjs.
• node --test .github/scripts/test_check_plugin_source_compatibility.mjs (9 tests).
• MCP app typecheck, generated-schema freshness, and formatting checks for the changed TypeScript files.
• Focused artifact-discovery and Deep Scan coordinator tests.
• Focused Python timestamp, archive, and Windows-file tests: 26 passed, 33 subtests passed; 5 Windows-only tests skipped on macOS.
• Bundled plugin build, using the native artifact from CI for the unchanged native source revision.
• Two independent native code reviews and an independent verification pass, with no actionable findings.

Risk and rollout

Small behavior-preserving cleanup. The Python imports retain their existing bindings, and the exception change only affects a test that expects SystemExit. Windows-specific behavior still needs the normal Windows CI coverage.

Public disclosure review

• No customer, partner, prospect, or user identities, data, or identifying details are included.
• No credentials, personal data, private source, scan findings, or nonpublic links or tickets are included.
• I reviewed the branch name, title, description, commits, changes, comments, logs, screenshots, attachments, and links for public disclosure.