Tests were self-skipping in CI for lack of a reachable Redis, hiding
most of the suite from the pipeline. Add a redis:7 service mapped to
127.0.0.1:6379 so the existing fsockopen probe in tests/Pest.php
passes and all 19 tests run.

Add Pint and PHPStan (level 8, no baseline or suppressions) as CI
gates. Larastan was tried first but its Testbench bootstrap crashes
under this package's fledge-fiber dependency (FiberHttpServiceProvider
calls Illuminate\Http\Client\Factory::globalHandler, which does not
exist outside a Fledge app), so plain phpstan/phpstan is used instead.

Fix the handful of level 6-8 findings: an inaccurate array-shape
docblock in MessageLog::decode(), an object cast in
ReplayBroadcaster::broadcast() narrowed to Stringable, and a nullable
RedisClient property re-read in MessageReplayPlugin::replay() that is
now passed in from the already-null-checked caller.