Completion-upload concurrency test must reconcile retryable SQLite responses

[rmcdaniel]

Main feature CI at 1c30eb1 failed in RuntimePayloadCompletionProcessTest for SQLite/activity: concurrent conflicting uploads returned a retryable 503 backend_lock_pressure and a 409 external_payload_completion_conflict, instead of the asserted initial 201 plus 409/503.

Failed run. This is separate from the activity deadline fix in Workflow 2.0.11; published Server 2.3.8 activity and replay qualification passed.

The completion path deliberately commits its bounded slot reservation before object storage/registration, so a later lock-pressure response can leave that reservation authoritative. A 503 is not proof that nothing committed. Inspect and prove cold retry of the same bytes before classifying this as a test assumption rather than a product defect.

Acceptance:

• Reproduce lock pressure after the reservation commit with real isolated SQLite state and fresh PHP requests.
• Prove conflicting bytes remain rejected, original bytes recover one reference, and the lease budget/object count never grows on retries.
• Keep strict MySQL/PostgreSQL concurrency assertions.
• Reconcile only the documented retryable lock response; do not accept arbitrary 503s or weaken durable-state checks.
• Restore main feature CI without a release/version change if only the fixture expectation is wrong.

[rmcdaniel]

Confirmed the production recovery path works. PR #155 reproduces the response boundary deterministically with a second real SQLite connection taking BEGIN IMMEDIATE after object commit. A fresh conflicting request returns 409; a fresh retry of the original bytes returns 201; reservation, object, reference and budget counts remain unchanged before and after workflow/activity completion.

The concurrency assertion now reconciles only SQLite 503/backend_lock_pressure/retryable=true once and then requires exactly [201, 409]. MySQL/PostgreSQL cannot use this allowance. No production change or new release is warranted by this test defect. Four local SQLite cases pass (90 assertions); native database coverage is running in the existing public feature job.