Guard action handle signal writes against stale submissions

[nakasyou]

Summary

When the same action handle is invoked concurrently (double-click, rapid re-invoke), invoke wrote result/error/lastSubmission/pending after awaits with no ordering guard, so an older submission resolving last would overwrite the newer result, and isPending flipped to false as soon as any submission settled.

Fix mirrors the version guard in signal.ts and the router's sequence check:

let latestSubmissionId = 0
const invoke = (input) => {
  const submissionId = ++latestSubmissionId
  ...
  applySubmissionChunk(submissionId, ...)   // each: if (submissionId !== latestSubmissionId) return
  applySubmissionError(submissionId, ...)
  finalizeSubmission(submissionId)          // only clears `pending` if still latest
}

The returned promise / async iterator still resolves with its own value; only the shared handle signals ignore superseded submissions. Single-submission behavior is unchanged.

Adds two unit tests in core/action.test.ts (fail before, pass after).

Link to Devin session: https://app.devin.ai/sessions/0cc06e7eec3c46019e2cca8c422b53de
Open in Devin Desktop: https://app.devin.ai/desktop/session/0cc06e7eec3c46019e2cca8c422b53de?variant=devin
Requested by: @nakasyou

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Commit: 49e5fc2

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