Preserve diagnostic pull state after quick document reopen

[Copilot]

A close-triggered diagnostic pull could asynchronously remove state created after the same URI reopened, preventing subsequent edits from requesting diagnostics.

• State management

  • Track pending forget operations with identity tokens.
  • Cancel pending cleanup when a matching document or notebook cell reopens.
  • Ignore stale pull callbacks without affecting normal close cleanup.

• Regression coverage

  • Simulate a language-ID change while the close-triggered pull is in flight.
  • Verify an edit after reopening still triggers a diagnostic pull.

• Fixes Diagnostics pull does not work if file is closed and reopened quickly #1821

[azure-pipelines]

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