Retry finalizing claimed executions on transient errors

[wintan1418]

Fixes #748

Problem

When a job completes, the worker finalizes it: a small transaction that deletes the ClaimedExecution and marks the Job finished (or failed). If that transaction raises — e.g. a transient DB connection drop, exactly the scenario reproduced in #748 — the error is swallowed by the pool's thread error handler and the claimed execution stays claimed forever:

• Every recovery mechanism (release on deregistration, failing orphaned executions on supervisor boot, failing pruned processes' claims) assumes the claiming process is gone.
• Here the worker is alive and healthy, so nothing ever picks the execution up again — even though the job already ran to completion.

Fix

Finalization is idempotent: it takes a row lock on the claimed execution and no-ops if it's already gone (unless_already_finalized). That makes it safe to retry. ClaimedExecution#perform now retries the finalization of both outcomes (finished and failed) through a few increasingly spaced attempts (0.5s, 1s, 2s, 4s) before giving up and letting the error propagate as before.

This handles transient instability, which is the case reported in #748 (and what its reproduction script simulates). A persistent DB outage is a different problem with different machinery (#549, #683): once nothing can talk to the database, heartbeats fail too and the process-pruning recovery path takes over when connectivity returns.

Each retry emits a retry_finalization.solid_queue event (logged at warn) with the job, process, attempt number, and error, so operators can see the instability instead of it being invisible.

Tests

• Finishing and failing paths each retry past a simulated transient ActiveRecord::ConnectionNotEstablished and finalize correctly on the next attempt.
• When retries are exhausted, the error propagates as before (no behavior change for persistent failures).
• Log subscriber coverage for the new event.

All three regression tests reproduce the stuck-claimed state on main and pass with the fix.