fix(extension): buffer early WebMCP.toolResponded events before command resolution (#2724)

[teddiesloco]

Summary

• Fixes WebMCP completion can be lost when toolResponded arrives before invokeTool resolves #2724
• In packages/extension/understudy/page.ts, buffer WebMCP.toolResponded events in earlyWebMCPResponses if they arrive before the WebMCP.invokeTool CDP command response settles.
• Flushes buffered completion event upon registration, resolving the race condition that causes waitForWebMCPInvocationResult() to hang indefinitely on fast tools.

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Summary by cubic

Buffers WebMCP.toolResponded events that arrive before the WebMCP.invokeTool CDP command settles, so fast tools no longer cause waitForWebMCPInvocationResult() to hang indefinitely.

Fixes #2724.

^{Written for commit e465ecb. Summary will update on new commits.}

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