fix(server): destroy stream in StreamDispatcher._onDispose

[SebTardif]

Summary

• StreamDispatcher does not override _onDispose(), so when the dispatcher hierarchy is torn down (client disconnect, page close), the underlying Readable stream from fs.createReadStream is never destroyed, leaking a file descriptor per open artifact stream.
• Every other resource-holding dispatcher overrides _onDispose() for cleanup: TracingDispatcher, PageDispatcher, DebugControllerDispatcher, WebSocketRouteDispatcher.

Origin

StreamDispatcher was introduced in #2733 (2020-06-26). The _onDispose() pattern was established later in #19572 (2022-12-19) by dgozman, which added _onDispose() to page, context, and tracing dispatchers, but missed StreamDispatcher. A subsequent stream fix in #27291 addressed event listener leaks in the same file but not the missing _onDispose().

[github-actions]

Test results for "MCP"

9 failed
❌ [firefox] › mcp/annotate.spec.ts:273 › should enter annotate mode on fresh dashboard.tsx mount with -s --annotate @mcp-windows-latest-firefox
❌ [firefox] › mcp/annotate.spec.ts:297 › should annotate via direct browser_annotate MCP call @mcp-windows-latest-firefox
❌ [firefox] › mcp/annotate.spec.ts:330 › should annotate when context has no fixed viewport @mcp-windows-latest-firefox
❌ [firefox] › mcp/annotate.spec.ts:367 › should cancel browser_annotate when the MCP request is aborted @mcp-windows-latest-firefox
❌ [firefox] › mcp/annotate.spec.ts:398 › should cancel browser_annotate when the MCP client disconnects @mcp-windows-latest-firefox
❌ [firefox] › mcp/annotate.spec.ts:427 › should switch screencast to -s session on show --annotate @mcp-windows-latest-firefox
❌ [firefox] › mcp/annotate.spec.ts:476 › should disengage annotate mode when --annotate client disconnects @mcp-windows-latest-firefox
❌ [firefox] › mcp/cli-devtools.spec.ts:217 › video-start-stop @mcp-windows-latest-firefox
❌ [firefox] › mcp/cli-devtools.spec.ts:231 › video-chapter @mcp-windows-latest-firefox

7048 passed, 1068 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

5 flaky

⚠️ [installation tests] › screencast.spec.ts:18 › screencast works `@package-installations-windows-latest`
⚠️ [chromium-library] › library/beforeunload.spec.ts:130 › should support dismissing the dialog multiple times `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/video.spec.ts:719 › screencast › should work with video+trace `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/video.spec.ts:275 › screencast › should capture navigation `@chromium-ubuntu-22.04-node24`
⚠️ [playwright-test] › ui-mode-trace.spec.ts:812 › should update state on subsequent run `@windows-latest-node20`

41701 passed, 850 skipped

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Merge workflow run.