What does this PR do?

Ensures Elastic Agent gracefully shuts down the EDOT (collector) subprocess when the agent receives a terminating signal (e.g., SIGTERM) - i.e., when the OTelManager context is cancelled - instead of immediately killing it.

Key changes:

• Replace collectorHandle.Stop(ctx context.Context) with Stop(waitTime time.Duration) and honor it in both embedded and subprocess execution modes.
• Set waitTimeForStop to 30s (aligned with Beats subprocess defaults).
• Add warnings when the supervised collector fails to stop gracefully or times out.
• Make updateCh channel buffered to a size of 1 and drain before write (pattern used elsewhere) to avoid shutdown/reconfig delays.
• Unit-Tests:
  • Extend test harness to allow configurable shutdown delays via TEST_SUPERVISED_COLLECTOR_DELAY.
  • Validate both outcomes:
    • Delay > 30s ⇒ subprocess is force-killed.
    • Delay < 30s ⇒ subprocess exits cleanly within the timeout.

Why is it important?

Previously, on shutdown the agent could terminate in a way that kills EDOT immediately, risking incomplete cleanup of telemetry pipelines and leaving resources in a bad state. Waiting (up to 30s) for a graceful EDOT exit improves cleanup, stability, and hybrid agent resilience.

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

No disruptive impact expected. The only behavior change is during shutdown: the agent now waits up to 30s for EDOT to exit gracefully before force-killing it. This improves stability and predictability for users..

How to test this PR locally

mage unitTest

Related issues

N/A