Published-SDK patch cohort: PHP SQLite poll refusal (backend_lock_pressure) ends the Python patch Worker #721

Finding

In the published-SDK patch cohort, the published PHP 2.11.1 SQLite runtime refused a Python patch Worker's workflow-task poll with a typed 503, and the Worker exited.

Observed in run 40017239524 / job 120644918290, the execution-and-codec job of PR #708 at head 6b6757e7e2. The same shape failed #707's execution-and-codec job (job 120591054917) in signals-before-worker.

• Every PHP, embedded and Rust fixture record in the step passed first, including all operator-metrics cases.
• scripts/conformance/server-parity/published-sdk-patch/python.py then started the published Python patch-replacement Worker against the shared published-PHP SQLite runtime.
• Its workflow-task poll got 503 with poll_status: backend_lock_pressure and PHP's message Worker poll backend is temporarily locked.. The published SDK raised ServerError from poll_workflow_task and the Worker exited.
• At the same time, the PHP maintenance container logged SQLSTATE[HY000]: General error: 5 database is locked on a TaskRepair update: 24 repairs completed and one candidate failed.
• The Worker's deregistration got one 503 and then succeeded on retry.

This is a real refusal from the reference, not an observer artifact. published-sdk-patch/observe-http.mjs deliberately never retries /poll backend_lock_pressure, even with DW_PARITY_PROXY_RETRY_LOCK_PRESSURE=1.

To decide

1. Cohort scheduling. The patch cohort runs PHP's maintenance loop and a polling Worker on one SQLite file. Should the cohort keep that contention, or give them a non-contending schedule, for example by pausing maintenance during the patch window? This must not hide the refusal.
2. SDK behavior. backend_lock_pressure is a typed, retryable poll refusal. The published Python SDK ends its Worker on it instead of backing off. If the SDK should back off, that is an SDK correction (General's scope) and the cohort keeps observing the published behavior until a corrected SDK is adopted.

Acceptance

• The observer never retries /poll lock pressure invisibly.
• Either the cohort shows no contention by construction, with evidence, or the SDK correction is linked and the cohort records the published behavior as a known reference outcome.
• Native runs are unaffected. This happens only on the published PHP reference.

Owner: Rust Server (cohort harness). SDK question routed to General.