Destroy orphan BlockedExecution rows when the job class no longer resolves

[julik]

Fixes #784.

Summary

Fixes a DelegationError raised on every SolidQueue::Dispatcher::ConcurrencyMaintenance tick when a BlockedExecution row references a Job whose class_name no longer safe_constantizes — e.g. because the concurrency-limited ActiveJob class was renamed or removed between deploys.

Job#acquire_concurrency_lock already gates on concurrency_limited? (which checks job_class.present?), but BlockedExecution#acquire_concurrency_lock — invoked from the dispatcher-driven release path — went straight to Semaphore.wait(job), which calls job.concurrency_limit, which is delegated to a nil job_class:

ActiveSupport::DelegationError: concurrency_limit delegated to job_class, but job_class is nil
  app/models/solid_queue/job/concurrency_controls.rb:11:in 'concurrency_limit'
  app/models/solid_queue/semaphore.rb:86:in 'limit'
  app/models/solid_queue/semaphore.rb:58:in 'attempt_creation'
  app/models/solid_queue/semaphore.rb:46:in 'wait'
  app/models/solid_queue/blocked_execution.rb:65:in 'acquire_concurrency_lock'
  app/models/solid_queue/blocked_execution.rb:49:in 'block (2 levels) in release'
  ...
  lib/solid_queue/dispatcher/concurrency_maintenance.rb:40

Because the exception is raised inside BlockedExecution#release's transaction, the row is never destroyed or promoted, and the dispatcher re-picks the same orphans forever — flooding error reporters and starving any legitimately blocked jobs that share the concurrency key.

Changes

• BlockedExecution#release: if job.job_class is nil, destroy the orphan row and emit orphaned: true in the release_blocked instrumentation payload. Otherwise fall through to the existing acquire/promote/destroy path.
• BlockedExecution#set_expires_at: fall back to SolidQueue.default_concurrency_control_period when the class does not resolve (guards the same latent bug at create time).
• Job#job_class promoted from private to public so BlockedExecution can consult it.
• New unit test covering the orphan release path.

Test plan

• bin/rails test test/models/solid_queue/blocked_execution_test.rb — new test passes.
• bin/rails test test/models/ — full model suite green (97 runs, 585 assertions, 0 failures).
• CI across mysql/postgres/sqlite.