Fix: propagate opentelemetry-instrument and ddtrace-run to flow run subprocesses

[shnhdan]

Closes #20535

Problem

When users run opentelemetry-instrument prefect flow-run execute, the
auto-instrumentation setup does not propagate to the subprocess that
actually runs the flow. This is because the child process is spawned
fresh and doesn't inherit the instrumentation bootstrapped in the parent.

Solution

The process worker now detects active wrapper commands via environment
signals and prepends them to the subprocess command:

• OTEL_* env vars → prepends opentelemetry-instrument
• DD_* env vars → prepends ddtrace-run

Changes

• Added _detect_active_wrappers() helper in process.py to detect
  active wrappers via env var signals
• Updated prepare_for_flow_run() to prepend detected wrappers to the
  subprocess command
• Added PREFECT_WORKER_PROCESS_COMMAND_WRAPPERS setting (default:
  ["opentelemetry-instrument", "ddtrace-run"]) so users can opt in/out
  or add custom wrappers by configuring this setting

Testing

• 11 unit tests covering all wrapper detection scenarios
• Verified manually via verify_otel.py

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing shnhdan:fix/otel-flow-run-subprocess (87e2978) with main (98309ff)}

[desertaxle]

Thanks for opening a PR @shnhdan!

As I mentioned on the linked issue, we'd prefer to use the setup_environment hook if possible to handle setting up OTel for the flow run execution process. Would you be able to try that approach instead?