Release Python SDK 2.3.1

[rmcdaniel]

Release the worker backend-outage recovery from #73. Version metadata moves to 2.3.1 and records Server 2.4.8 as the exact qualified Server release. No wire protocol change. The release-metadata test now validates the version shape rather than duplicating a hard-coded Server patch number.

Source qualification: merged SDK commit c0678f20ed714a02e9bb67b910eecff6892441bb ran against published Server 2.4.8 image digest sha256:692f6db63f6130342fc6e3bc13ffb159bca99f854d28872e0059432aabc95f68 in a disposable MySQL/Redis cell. One long-lived worker completed a baseline workflow, survived repeated structured 503s during Redis loss and MySQL loss with original PID and zero restarts, and completed workflows after each backend recovered. The current release branch passes 1,450 non-integration tests locally; CI is running.

Release follow-through: verify CI and built metadata; merge; publish immutable 2.3.1 tag through the repository workflow; install the exact PyPI artifact and repeat the backend-loss cell; record results and close #72 only after published verification. Server, Workflow, PHP/Rust SDKs, and Cloud require no code release for this Python-only retry-policy change. Sample App retains its separately qualified older SDK tuple until its own published-artifact requalification, rather than silently changing a pin here.

[rmcdaniel]

Release follow-through complete: 2.3.1 is tagged at f03d3f63a9fe22b6cd7e2828bf605c542ba9b104; publication and exact PyPI metadata checks passed. The installed PyPI wheel was exercised against published Server 2.4.8 through separate Redis/MySQL outages; sdk-python #72 records the artifact hashes, result, and claim limits. No Server, Workflow, PHP SDK, Rust SDK, or Cloud code release is required for this SDK-only patch. Sample App remains on its separately qualified tuple; it was not silently repinned by this release.