Prevent poll backpressure from starving other task kinds

[rmcdaniel]

Addresses #66.

The PHP worker can remain in one task-kind retry loop indefinitely after a typed HTTP 429 long_poll_capacity_exhausted refusal, starving activities and queries despite successful heartbeats.

After the existing interruptible backoff, return that validated empty-task refusal to tick() so the other task kinds get a turn. No lease was acquired. Ambiguous connection/database/storage failures retain their existing retry identity and handling; no Server admission setting or wait deadline changes.

Verification

• Published 2.0.1 and 2.0.9 negative controls: 17 workflow polls, 11 heartbeats, zero activity/query polls over 60 simulated seconds.
• Five new liveness cases fail against the unmodified worker; workflow/activity/query/all refusal directions and activity completion pass with the fix.
• Shutdown during capacity backoff starts no further poll.
• Full PHP 8.3 suite: 639 tests, 3,673 assertions, one existing live-runtime memo test skipped because no runtime URL was supplied.
• PHPStan passes.
• Python and Rust source inspection shows independent managed polling tasks; Rust also settles an explicit capacity refusal as no task. This is source inspection, not a new runtime conformance pass.

Prepares PHP SDK 2.0.10. Publication, installed-package regression, published service-mode smoke and the Server soak consumer update remain required before issue closure. The short local Server run completed 60/60 workflows without reproducing the six paid-run timeouts; this fix is not claimed as their complete diagnosis. Server investigation: durable-workflow/server#137.

[rmcdaniel]

Published follow-through: 2.0.10 is installable from Packagist, source 19b2695. Fresh published-consumer smoke passed: https://github.com/durable-workflow/sdk-php/actions/runs/34616341682 (Server 2.1.0). The installed-package deterministic backpressure reproduction also passed: 60 simulated seconds, 60 workflow polls, 59 activity polls, 59 query polls, 11 heartbeats, graceful shutdown. Both installed older negative controls had zero activity/query polls. The affected Server consumer update is durable-workflow/server#161; no Server runtime dependency or unchanged Python/Rust release is required. The original paid-soak timeout investigation remains separate.