Implement trace context initialization in Rails for CI job telemetry

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Summary

Implement trace context initialization in Rails so that all telemetry spans for a CI job share the same trace_id. The trace context is passed to the Runner in the job payload response when job_telemetry is enabled.

Job payload change

Add a trace_context object to the /api/v4/jobs/request response (only when job_telemetry is enabled):

{
  "trace_context": {
    "trace_id": "00000000000000000000000002a6c18e",
    "parent_span_id": "a1b2c3d4e5f60718"
  }
}

Field	Type	Description
trace_id	String (32-char hex)	Deterministically derived from job_id. All spans for this job share this trace ID.
parent_span_id	String (16-char hex)	The span ID of the Rails-created job_lifecycle span (Phase 3). For MVC this is a placeholder root — the Runner's job_execution span becomes the trace root and will be re-parented once Phase 3 adds the Rails job_lifecycle span.

Requirements

1. Generate trace_id deterministically from job_id (e.g., zero-padded hex representation, or UUID v5)
2. Generate parent_span_id as a random 16-char hex span ID (placeholder for MVC; becomes the actual job_lifecycle span ID in Phase 3)
3. Only include trace_context when the job_telemetry feature is enabled (see #590588)

Architecture Reference

• Job payload changes
• Multi-source trace context coordination

Dependencies

• Feature negotiation (#590588) must be implemented first — trace_context is only included when job_telemetry is enabled
• Coordinate with grouprunner core on job payload schema changes (&20633)

Parent Epic

&20945