refactor: make the pending-subscription sentinel explicit

[blaipr]

WSClient._should_subscribe_to_pending_job holds False until a pending subscription is queued and a dict afterwards, and three places subscript it. The subscripts are safe, because the only path that reaches them tests the flag first, but the test was buried:

if all([message.get('group_name') == 'jobs', message.get('status') == 'pending', message.get('unified_job_id'), self._should_subscribe_to_pending_job]):
    if bool(message.get('project_id')) == (self._should_subscribe_to_pending_job['events'] == 'project_update_events'):
        self._update_subscription(message['unified_job_id'])

Four unrelated conditions in an all([...]), one of them the guard for the line below it, and _update_subscription then reaching back for the attribute a second time rather than being handed it.

Three changes, none of which alters behaviour:

• The sentinel is None rather than False, annotated dict | None. Both are falsy and nothing compares it by identity or to False, so every existing check behaves the same. None is what "not set yet" means.
• The flag is bound to a local and tested first, with and instead of all([...]). Short-circuiting rather than eager evaluation, which is fine here since every element is a pure .get().
• _update_subscription takes the dict as an argument instead of re-reading the attribute, so it cannot be called in a state where that attribute is unset.

Exercised the whole path directly, since the unit suite covers the callbacks but not this branch: queueing with subscribe_to_pending_events('job_events'), then feeding a pending-job message through _on_message, resubscribes with {'jobs': ['status_changed'], 'job_events': [7]} and clears the sentinel back to None.

Four diagnostics retired. Diffed the full list before and after: strict subset, nothing introduced.

Note for whoever merges: #27 also edits this file, so whichever lands second needs a rebase.

Verified with black --check, flake8 and the unit suite, 355 passing.

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