Backport startup diagnostics to stable Python 2.5.8

[rmcdaniel]

Stable backport

Backport #126 onto published 2.5.7 and prepare 2.5.8. A startup timeout now identifies the worker, workflow and existing wait budget. Discovery/registration regressions and the post-startup timeout control execute without changing deadlines, polling or cleanup authority.

Main contains the unqualified 2.6.0 Nexus producer candidate. This branch includes only the reviewed diagnostic, its tests/docs and aligned 2.5.8 package declarations. No producer API is included.

The equivalent main change passes all normal checks, including real HTTP integration. The stable branch receives independent source/package qualification, then exact registry and published Server verification. #123 stays open because the original unconstrained registration delay remains unexplained.

[rmcdaniel]

Python 2.5.8 is published from b30d38fa2e457a361a1a955eabd1e51e1c28c0b7, the narrow stable backport in #127. All stable source checks and publication/registry checks pass.

Fresh installed worker code matches that release source byte-for-byte. Both pinned and default PyPI resolution select 2.5.8. Wheel SHA256 6873d8e5473aa457c292b76b6e8960876d1513f6da601cbf7ca4c6e013e44e9d, sdist SHA256 ff15015635f5abd652f4d71e850f4acccf62a7456f2f25e97b4f26499212ad40.

The real HTTP delayed-registration check uses unmodified published Server 2.6.1 at sha256:ef043e5c582761e486a64a2ab4226476e17ae4f71682ef59d6b9ddd784d246b6 and the SDK's actual default 1.20. A two-second held reply completes normally in 5.30 seconds. With a 35-second held reply after Server HTTP 201, the new startup error occurs at 30.03 seconds under the unchanged 30-second wait budget, includes worker/workflow identities and preserves its cause. No task polling or deregistration occurs before acknowledgment. A replacement completes the original run once with the correct timer result.

The original unconstrained registration delay remains unexplained, so #123 stays open. This release improves diagnosis without extending budgets or shipping the unqualified producer APIs from main. The live worker documentation contains the new explanation. Sample App #205 is the remaining pinned-image follow-through. Other package and Server consumers do not require unchanged releases for this Python-only diagnostic. Cloud and Rust retain their frozen qualification tuples and decide their own adoption.