fix(reliability): prevent webview postMessage crashes and make dispose idempotent

[0xMink]

Closes: #11311

Summary

• Idempotent dispose(): Added _disposed guard so repeated calls (e.g. during rapid extension deactivation) are no-ops.
• Catch postMessageToWebview() rejections: Wrapped webview.postMessage() in try/catch so messages sent after the webview is disposed do not surface as unhandled promise rejections.
• Tests + mock maintenance: Two new tests covering both fixes. Updated the CloudService mock to include off() — a pre-existing gap exposed by the new dispose test.

Test plan

• postMessageToWebview does not throw when webview is disposed
• dispose() is idempotent — second call is a no-op
• All 90 existing tests still pass (6 skipped, unchanged)

Roadmap alignment: Reliability First

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Important

Make dispose() idempotent and handle promise rejections in postMessageToWebview() in ClineProvider to improve reliability.

• Behavior:
  • Make dispose() in ClineProvider idempotent by adding _disposed flag to prevent repeated execution.
  • Wrap postMessageToWebview() in ClineProvider with try/catch to handle promise rejections when webview is disposed.
• Tests:
  • Add test for postMessageToWebview to ensure no error is thrown when webview is disposed.
  • Add test for dispose() to verify it is idempotent and logs only once.
• Mocks:
  • Update CloudService mock to include off() method in ClineProvider.spec.ts.

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[roomote]

Rooviewer   See task

Reviewed the changes. No issues found -- the _disposed guard on dispose() and the try/catch in postMessageToWebview() are clean, minimal fixes that correctly address the reported problems. Tests cover both behaviors.

• No issues to resolve

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