Optional disabling of ClusterIP routing in the activator

[elijah-rou]

Adds the ability to optionally disable ClusterIP routing on a per-revision basis through the 'serving.knative.dev/disable-clusterip-routing' annotation.

When this annotation is set to 'true' on a revision, the activator will bypass ClusterIP routing and use direct pod IP routing exclusively, even when a healthy ClusterIP is available.

Key changes:

• Add DisableClusterIPRoutingAnnotationKey constant for the new
  annotation
• Extend revisionThrottler to track and respect the disableClusterIP
  setting
• Modify routing logic to skip ClusterIP when disabled
• Read annotation value during throttler creation from revision metadata
• Maintain full backward compatibility (ClusterIP routing enabled by
  default)

This feature is useful for scenarios where direct pod routing is preferred over ClusterIP load balancing, such as for debugging, performance testing, or when specific routing behaviors are required.

Includes comprehensive tests to verify both disabled and default behaviors.

[knative-prow]

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[knative-prow]

Hi @elijah-rou. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.17%. Comparing base (973fcba) to head (9c8f884).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16081      +/-   ##
==========================================
+ Coverage   80.14%   80.17%   +0.03%     
==========================================
  Files         214      214              
  Lines       16877    16883       +6     
==========================================
+ Hits        13526    13536      +10     
+ Misses       2989     2985       -4     
  Partials      362      362              

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[dprotaso]

You might want to take a look at #10751

The intent was to remove the cluster IP routing completely. It's there because of Istio Mesh.

But back then I discovered our mesh tests weren't actually setting up mesh properly. That's since been fixed but Markus left the project before revisiting the work.

I think I'd prefer to revisit that with the hope of removing these fallbacks completely.

Also supposedly this isn't needed in Istio ambient mode at all.