fix(mcp extension): look for extension profile in default user data dir

[sazzadsadib]

fixes the silent hang from microsoft/playwright-mcp#1571

running with --extension and no --user-data-dir, the relay spawns chrome without --profile-directory, so connect.html opens in the last used profile. if the extension lives in a different profile the page never loads and establishExtensionConnection() waits forever with no error.

this adds a defaultUserDataDir(channel) helper returning the standard chrome/edge user data folder per OS, used as fallback so the existing profile lookup actually runs on default setups. no behavior change if you pass your own dir. unknown channels fall back to old behavior.

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