Extracted _close_vscode_diff_terminate_result(app) -> Result[None]
helper above the App._close_vscode_diff method.
ANTI-SLIMING: full Result[T] propagation (NO except+pass). The helper
returns Result(data=None) on success or Result(data=None,
errors=[ErrorInfo]) on exception (logging NOT a drain per the user's
principle 2026-06-17).
The legacy _close_vscode_diff method preserves its signature, calls
the helper, drains errors to self._last_request_errors, and proceeds
to set self._vscode_diff_process = None (preserving the original
post-error behavior of clearing the handle).
Tests: 2 new tests verify both paths (success and OSError).
Audit: L1466 reclassified from INTERNAL_SILENT_SWALLOW (7 sites remaining,
was 8). New helper L1466 is INTERNAL_COMPLIANT.
ed
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