Optimize runner manager heartbeat to skip unnecessary database updates

What does this MR do and why?

Optimizes the Ci::RunnerManager#heartbeat method to prevent unnecessary database writes when runner attributes haven't changed.

Runners send frequent heartbeat requests, and previously every heartbeat would trigger a database update even when only contacted_at changed (which is tracked in Redis cache). This MR adds a changed? check before calling update_columns, reducing database load for large runner fleets with stable configurations.

Changes:

• Modified app/models/ci/runner_manager.rb: Added && changed? condition to skip update_columns when no attributes have changed
• Added test coverage for no-change scenarios, cache-only updates, and partial attribute changes

References

• Issue: https://gitlab.com/gitlab-data/analytics/-/issues/25129+s
• Related: Runner heartbeat instructs postgres to UPDATE f... (#525577) • Unassigned • Next 4-6 releases

Screenshots or screen recordings

N/A - Backend optimization

How to set up and validate locally

1. Run the new specs to verify the optimization:

   bundle exec rspec spec/models/ci/runner_manager_spec.rb

2. To observe the behavior in development:

   # In rails console
   runner = Ci::Runner.first
   manager = runner.runner_managers.first
   
   # Heartbeat with no changes - should not update DB
   manager.heartbeat({ version: manager.version })
   
   # Heartbeat with changes - should update DB
   manager.heartbeat({ version: '16.0.0' })

MR acceptance checklist

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