fix: restore activity timeout failures during replay

[rmcdaniel]

Problem

Terminal activity timeouts record a class that did not exist. The next workflow task blocks on failure restoration instead of running catch/finally code.

Change

• Add the existing serialized ActivityTimeoutException identity as a concrete RuntimeException subclass.
• Use its class constant in all three timeout producers without changing persisted identity or classification.
• Cover persisted failures without application mappings, ordinary/local activity catch and cleanup, and uncaught terminal failures.
• Add a cold-replay corpus fixture.

Verification

• Focused unit/regression suite: 47 tests, 1,000 assertions pass (PHP 8.4.25, disk-backed SQLite).
• Counterfactual against exact base e4146a82: all six new regression cases fail, including missing-class restoration and skipped cleanup. The candidate passes them.
• ECS and full PHPStan pass.
• Full main build passed, including database feature matrices and coverage.
• Published-package verification passed for 2.0.9, including the supported Laravel upgrade matrix.

Fixes #498.