Add job_telemetry feature negotiation to CI job request

What does this MR do and why?

Adds job_telemetry: true to the features object in the /api/v4/jobs/request response, gated behind the ci_job_telemetry beta feature flag.

This is the Rails-side implementation of feature negotiation for CI job telemetry. When the feature flag is enabled for a project, the runner receives job_telemetry: true in the job payload, signaling it to emit telemetry data.

• Relates to #590588 (closed)
• Feature flag rollout issue: #590939

Implementation details

• EE override of Ci::Build#features in ee/app/models/ee/ci/build.rb — conditionally adds job_telemetry: true when Feature.enabled?(:ci_job_telemetry, project)
• Beta feature flag ci_job_telemetry — disabled by default, for gradual rollout
• No changelog — change is behind a disabled-by-default feature flag

How to set up and validate locally

1. Enable the feature flag for a project:

   Feature.enable(:ci_job_telemetry, Project.find_by_full_path('your/project'))

2. Register a runner and request a job via the API or trigger a pipeline
3. Verify the job payload includes "features": { ..., "job_telemetry": true }
4. Disable the flag and verify job_telemetry is absent from the response:

   Feature.disable(:ci_job_telemetry)

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