Fix memqueue race condition during shutdown

[faec]

The memory queue had a bug where if Close was called on an already-empty queue, it would mark itself as closing but never cancel the final internal context. The main effect was that Get requests to such a queue would still block instead of correctly returning io.EOF immediately. This caused some test flakiness, e.g. TestClient/no_infinite_loop_when_processing_fails depended on the io.EOF return value to end the test, but whether Close was called before or after draining the queue depended on goroutine execution order. This PR moves the context cancellation check to the end of the run loop iteration, so it's updated after every state change and not just after event deletions. The accompanying unit test fails without this change.

Checklist

• 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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @faec? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.