fix(mcp): use bundled Chromium for --browser chromium

[Swigler]

Summary

• --browser chromium currently sets channel: 'chrome-for-testing', but that channel is not registered as a launchable executable in the browser registry — it only exists as an alias for playwright install. This causes a runtime error: Chromium distribution 'chrome' is not found at /opt/google/chrome/chrome
• Fix: drop the channel entirely so Playwright uses the bundled Chromium binary directly (same pattern as --browser firefox)
• Add chromium to the --browser help text, which previously omitted it

Changes

• packages/playwright-core/src/tools/mcp/config.ts — remove channel: 'chrome-for-testing' from the chromium case in resolveBrowserParam()
• packages/playwright-core/src/tools/mcp/program.ts — add chromium to valid values in help text
• tests/mcp/config-resolve.spec.ts — update 3 tests to expect no channel for chromium

Test plan

• Verified locally: --browser chromium launches bundled Chromium without error
• Navigated to multiple sites (example.com, youtube.com, facebook.com) successfully
• Upstream test suite (config-resolve.spec.ts)

🤖 Generated with Claude Code

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