Stabilize the MCP teardown deadline test

[Widthdom]

Summary

• keep both counted in-flight requests behind cancellation-insensitive gates until teardown diagnostics are captured
• verify shutdown cancellation separately without making the asserted request count depend on cancellation callback scheduling
• document the deterministic teardown-test contract in English and Japanese
• add the bilingual changelog fragment changelog.d/unreleased/4653.internal.md

Root cause

The isolated Release test counted a cancellation-responsive task as unfinished. Once shutdown cancellation started, whether that task was pruned before the zero-length final deadline depended on callback scheduling, so the final diagnostic intermittently reported a different state or was absent.

Validation

• exact isolated Release test: passed on .NET 8 and .NET 9
• 10 repeated isolated Release runs: passed on both target frameworks (20/20)
• focused DrainInFlightTasksAsync_* tests: 7/7 passed on each target framework
• full Release solution tests: 9,743 passed / 6 skipped on .NET 8; 9,278 passed / 410 skipped on .NET 9
• Release solution build: 0 warnings, 0 errors
• make lint
• changelog fragment validation
• Codex adversarial review: No blocking/actionable issues found.

Fixes #4653