fix(dev): await worker shutdown before replacing it - #4506

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/dev/server.ts`:
- Around line 235-237: Update the catch block around the shutdown message
delivery to capture the send error and issue a warning containing the shutdown
operation context and error message, then continue calling done() as before.
- Line 155: Update close() before the await this.#shutdownWorker() call to set a
permanent closing state, reject both new and already queued reload() operations,
and await any active reload represented by `#reloadPromise` before invoking
`#manager.close`(). Ensure reload cannot call `#manager.reload` after closing
begins, while preserving the existing shutdown and watcher-close sequence.
- Around line 82-84: Update the shutdown flow in `#shutdownWorker`() and local
close() so RunnerManager cleanup occurs before `#shuttingDown` is cleared, or keep
that flag set through manager.close(). Ensure registered onClose listeners
observe the intentional-shutdown state and do not call reload(), while
preserving listener cleanup ordering.

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Nitpick comments:
In `@src/dev/server.ts`:
- Around line 211-243: Extract the worker shutdown coordination logic from the
server.ts file into a new internal module such as src/dev/_worker-lifecycle.ts.
Move the `#shutdownWorker` method, the `#shuttingDown` state field, and related
shutdown protocol constants like SHUTDOWN_TIMEOUT into the new module. Create a
class or exported functions in the new module that encapsulate this lifecycle
management, then update the server.ts file to use the extracted module, removing
the original `#shutdownWorker` method and `#shuttingDown` field while delegating to
the new module.