Name transport failures and local cancellation in worker logs and request metrics #167

Python worker logs and request metrics do not let operators tell an intermediary disconnect or client timeout apart from a server refusal or a local cancellation.

Current behavior

Main 150c6f6bc14e3516f75cc615b2f75f093ed84ea1:

• _poll_error_summary returns str(error) for anything that is not a ServerError. Poll warnings log only that summary, with no exception class. httpx timeout and read errors can have an empty message, so the warning can name nothing.
• The request metric starts with outcome = "pending" and updates it only for Exception. asyncio.CancelledError is not an Exception, so a request cancelled by shutdown or by the caller is recorded with outcome="pending" and status_code="none".
• The Python docs do not list the outcome values or say that transport failures surface as raw httpx exceptions after retries run out.

This was found by reading source and the pinned dependency code. It was not reproduced against a live proxy.

Expected

Poll and request failure logs always name the exception class and route. Local cancellation has an explicit documented outcome. The docs separate transport failures, durable activity or workflow cancellation, and asyncio.CancelledError.

Acceptance

• Unit tests: a message-less ReadTimeout or ReadError produces a warning that names the class, and server refusals keep their status and reason.
• Cancelling an in-flight request records the documented cancellation outcome.
• Document the metric outcome values and the error categories.
• Decide whether to retry RemoteProtocolError for idempotent worker requests, document the decision and test it.
• Link from durable-workflow.github.io#267.