fix(server): destroy stream in WritableStreamDispatcher._onDispose

[SebTardif]

Summary

• Add _onDispose() to WritableStreamDispatcher to destroy the underlying fs.WriteStream when the dispatcher is disposed
• Matches the existing pattern in StreamDispatcher (fix(server): destroy stream in StreamDispatcher._onDispose #40759)
• Prevents file handle leaks when the client disconnects without calling close()

Introduced in #12860 (2022-03-18). The _onDispose() method was never added.

[github-actions]

Test results for "MCP"

7057 passed, 1068 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

3 flaky

⚠️ [chromium-library] › library/video.spec.ts:719 › screencast › should work with video+trace `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [chromium-library] › library/video.spec.ts:682 › screencast › should capture full viewport on hidpi `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/video.spec.ts:719 › screencast › should work with video+trace `@chromium-ubuntu-22.04-node22`

41717 passed, 850 skipped

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Merge workflow run.