feat(rails): delegate ActiveJob to backend extensions

[solnic]

Updates the ActiveJob integration to use the job extension API and shared producer/consumer instrumentation for backend-specific metadata, retry and error-reporting decisions, and trace propagation, while preserving defaults when no extension supplies an override.

Includes ActiveJob queue conformance and coverage for payload serialization, body size, and unsampled telemetry. Sidekiq's extension and ActiveJob handoff follow in the next PR.

[cursor]

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