fix(cua): make session.stop() idempotent on repeated call

[masnwilliams]

Summary

Stacks on top of the unified CUA template branch. Addresses the new High-Severity Cursor Bugbot finding on PR #143's latest scan: stop() throws on repeated call instead of being a no-op.

Background

PR #155 moved the session-state reset (_sessionId = null, …) into the finally so that a thrown replay-stop or browser-delete error wouldn't leave stale state behind. That fix was correct, but it exposed a latent bug: stop() opens with const info = this.info (TS) / info = self.info (Py), and the info getter delegates to the sessionId property, which raises when _sessionId is null. So once PR #155 reliably cleared _sessionId on the first call, the caller's error-path retry would hit the throwing getter and mask the original exception — exactly the failure mode PR #155 was meant to prevent.

Fix

stop() now:

1. Short-circuits at the top with a sentinel SessionInfo when no session is active — never touches the throwing getter.
2. Builds the live-session info from local fields directly so the body never depends on this.info / self.info either.

Symmetric in TS (session.ts) and Python (session.py).

Test plan

• go build ./...
• go test ./pkg/create/...
• Force a replay-stop failure (e.g. delete the replay out-of-band, or pass an invalid replay id) and confirm calling session.stop() twice from the caller's error path no longer raises and the original error surfaces.

---

Note

Medium Risk
Touches session shutdown/cleanup logic in both Python and TypeScript templates; while small, mistakes could lead to leaked browser sessions or masked errors during teardown.

Overview
KernelBrowserSession.stop() in both pkg/templates/python/cua/session.py and pkg/templates/typescript/cua/session.ts is updated to be idempotent.

It now short-circuits when no active session exists (returning a sentinel SessionInfo without reading info/sessionId), and builds the returned SessionInfo directly from internal fields so teardown can’t fail due to accessing a getter after state has been cleared.

^{Reviewed by Cursor Bugbot for commit db0bdea. Bugbot is set up for automated code reviews on this repo. Configure here.}

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