Release lock for interruptable client requests

[mishushakov]

fixes #213

[cursor]

PR Summary

Medium Risk
Changes the server-side execution locking scope in template/server/messaging.py, which affects concurrency and cleanup ordering; mistakes could cause leaked executions or out-of-order env var cleanup under load. Coverage is improved via new regression tests across JS and Python clients for the timeout/disconnect scenario.

Overview
Fixes issue #213 by releasing the websocket execution lock after the request is sent, rather than holding it through result streaming in template/server/messaging.py, so a client-side timeout/disconnect can’t block subsequent executions.

Adds regression tests in JS and Python (sync + async) to ensure a timed-out run doesn’t prevent later runCode/run_code calls in the same context, and includes a patch changeset for the template release.

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