feat(test-runner): add Reporter.plan() hook for test filtering

[Skn0tt]

Adds a Reporter.plan(rootSuite) hook that runs between onConfigure and onBegin, letting reporters filter/modify tests using skip/fixme/fail/exclude disposition methods on TestCase and Suite.

Reporters can also opt out of Playwright's built-in sharding by implementing implementsSharding().

Follow-up: expose Suite.parallelMode publicly so third-party sharders can keep test.describe.serial(...) chains together. Without it, custom sharding can only safely split tests that aren't in serial groups.

Fixes #40934

[github-actions]

Test results for "MCP"

7230 passed, 1103 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

4 flaky

⚠️ [chromium-library] › library/popup.spec.ts:261 › should not throw when click closes popup `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/video.spec.ts:647 › screencast › should capture full viewport `@chromium-ubuntu-22.04-node22`
⚠️ [firefox-page] › page/page-emulate-media.spec.ts:144 › should keep reduced motion and color emulation after reload `@firefox-ubuntu-22.04-node20`
⚠️ [playwright-test] › ui-mode-test-filters.spec.ts:75 › should filter by status `@windows-latest-node20`

39543 passed, 775 skipped

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Merge workflow run.