DEBUG-3573 Add integration test for telemetry app-heartbeat event payloads

[p-datadog]

What does this PR do?

Adds an integration test for AppHeartbeat telemetry event with payload assertions

Motivation:
Debugging why telemetry is not working properly for dynamic instrumentation - the UI requires telemetry heartbeats to show correct information but these are not seen by the backend

Change log entry
None

Additional Notes:

This PR duplicates the mark_telemetry_started helper from #4640

How to test the change?
PR is tests only

[github-actions]

Thank you for updating Change log entry section 👏

^{Visited at: 2025-05-13 23:14:56 UTC}

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-05-14 14:20:42

Comparing candidate commit 2d65e46 in PR branch telemetry-heartbeat-test with baseline commit f927b70 in branch master.

Found 1 performance improvements and 2 performance regressions! Performance is the same for 28 metrics, 2 unstable metrics.

scenario:profiling - gvl benchmark samples

• 🟥 throughput [-601.227op/s; -592.728op/s] or [-5.212%; -5.139%]

scenario:tracing - Propagation - Trace Context

• 🟩 throughput [+3558.068op/s; +3666.560op/s] or [+10.578%; +10.900%]

scenario:tracing - Tracing.log_correlation

• 🟥 throughput [-8373.041op/s; -8107.294op/s] or [-7.695%; -7.451%]

[datadog-datadog-prod-us1]

Datadog Report

Branch report: telemetry-heartbeat-test
Commit report: 2d65e46
Test service: dd-trace-rb

✅ 0 Failed, 21239 Passed, 1462 Skipped, 3m 51.64s Total Time

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.77%. Comparing base (f927b70) to head (2d65e46).
Report is 328 commits behind head on master.

Files with missing lines	Patch %	Lines
spec/support/telemetry_helpers.rb	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4643      +/-   ##
==========================================
- Coverage   97.77%   97.77%   -0.01%     
==========================================
  Files        1418     1419       +1     
  Lines       86587    86624      +37     
  Branches     4396     4397       +1     
==========================================
+ Hits        84662    84693      +31     
- Misses       1925     1931       +6     

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[Strech]

I like the changes, but I do have concerns over mutating "run once" constant, I would love to see it as a stub, over the mutation