fix(rails): Only patch background worker when ActiveRecord::Base is defined - #3170

Description

Sentry::Railtie applies Sentry::BackgroundWorker::ActiveRecordConnectionPatch whenever ActiveRecord is defined:

patch_background_worker if defined?(ActiveRecord)

The patch then calls ActiveRecord::Base in an ensure after every job:

ensure
  if ActiveRecord::Base.connected?
    ActiveRecord::Base.connection_pool.release_connection
  end

Some apps load only part of ActiveRecord, for example require "active_record/errors" or require "active_record/validations" for the error and validation classes, without active_record/railtie or require "active_record". In those apps ActiveRecord is defined, but ActiveRecord::Base is neither defined nor registered for autoload. So every background-worker job raises NameError in the ensure, and BackgroundWorker#perform logs:

exception happened in background worker: uninitialized constant ActiveRecord::Base

Events are still delivered, because the block runs before the ensure. But the SDK logs an error for every envelope it sends.

This PR checks defined?(ActiveRecord::Base) instead. defined? returns "constant" for the autoload that require "active_record" registers, and it doesn't load active_record/base, so apps with full ActiveRecord still get the patch with no change in load order.

Related: #1618 (ActiveRecord loaded but misconfigured, which led to the connected? guard). That isn't the same problem: here ActiveRecord::Base isn't loaded at all.

Tests

Adds spec/isolated/background_worker_spec.rb. The patch is applied at most once per process, so each case boots a bare app in a child process, like streaming_reporter_spec.rb:

• With active_record/railtie: the patch is applied, and a job runs without logging an error.
• With only active_model + active_record/errors: the patch is skipped, and a job runs without logging an error. Without the railtie change this case fails with "patched,ran,error".

The existing connection-release specs in spec/sentry/rails/client_spec.rb still pass for full ActiveRecord apps.

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