Add support for Solid Queue

[lairtonmendes]

What does this pull request do?

Adds support for Solid Queue, the
Active Record–backed queue adapter that became the default in Rails 8.
The new spy instruments SolidQueue::ClaimedExecution#perform and produces one APM transaction per job:

• transaction.name = the job's class name (the wrapped ActiveJob)
• transaction.type = SolidQueue
• label :queue = the queue the job was claimed from
• exceptions raised by the job are reported via ElasticAPM.report and mark the transaction outcome=failure

Implementation notes worth surfacing for review:

• SolidQueue::ClaimedExecution lives under app/models and is autoloaded by the Rails engine via Zeitwerk, so the spies' Kernel.require hook never fires. The prepend is installed inside Rails.application.reloader.to_prepare, which both delays installation until the engine's autoload paths are in place and re-applies the patch after dev-mode reloads.
• Fork mode (solid_queue's default supervisor) is already handled by Agent#detect_forking!, which runs on every start_transaction / start_span / report and rebuilds the agent's background threads in the forked child. No spy-side lifecycle hooks are needed.
• Gemfile gates gem 'solid_queue' to Rails ≥ 7.1 so the older Rails matrix entries keep resolving.

Why is it important?

Solid Queue is the default Active Job adapter in Rails 8 and is the queue shipped by rails new for every new Rails app from 8.0 onward. Without this instrumentation, jobs executed by solid_queue produce no APM transactions, leaving an entire class of work invisible in Kibana.

Checklist

• I have signed the Contributor License Agreement.
• My code follows the style guidelines of this project (rubocop clean on the new files)
• I have rebased my changes on top of the latest main branch
• I have added tests that prove my fix is effective or that my feature works (spec/elastic_apm/spies/solid_queue_spec.rb, 6 examples covering success, failure, queue label, multi-perform, prepend, and graceful no-op when agent is not running)
• New and existing unit tests pass locally (bundle exec rspec spec/elastic_apm/spies/ → 110 examples, 0 failures)
• I have made corresponding changes to the documentation
• I have updated docs/release-notes/index.md
• I have updated docs/reference/supported-technologies.md

Verification

End-to-end smoke test against a Rails 8.1 app using solid_queue as the
Active Job adapter, with traces captured by a mock APM server:

• async supervisor mode — all enqueued jobs produced one transaction each (success, failure with reported exception, non-default queue label).
• fork supervisor mode (solid_queue's default) — same coverage; each forked Worker process produces transactions, handled by the existing Agent#detect_forking!.

Related issues

N/A

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[lairtonmendes]

The finalizer registered via ObjectSpace.define_finalizer in proxy_pipe.rb calls io.close, which Ruby forbids inside a signal trap context (ThreadError). SolidQueue workers install persistent Signal.trap handlers; if GC runs while a trap is active (triggered by normal allocations mid-job), the finalizer fires in that context and Ruby
prints a warning. Fixed by rescuing ThreadError in the finalizer the fd is reclaimed by the OS on process exit, so skipping silently is safe