Fix #16260: Allow unreachable revisions with initialScale > 1 to scale to 0

[aviralgarg05]

Revisions with initialScale > 1 that are no longer referenced by any Route (routingState = 'reserve', mapped to Reachability = Unreachable) were previously unable to scale down to 0, causing resource waste when old revisions were replaced by new ones.

This change modifies the initialScale logic in scaler.go to ignore the initialScale constraint when a revision's Reachability is Unreachable. This allows revisions in reserve state to scale down to 0 immediately, freeing up resources promptly when they are no longer needed.

The fix adds a check for pa.Spec.Reachability != ReachabilityUnreachable before applying the initialScale constraint, ensuring that:

• Revisions with routingState = 'reserve' can scale down to 0 immediately
• The initialScale logic only applies to revisions that are still referenced by Routes (routingState = 'active')
• Resources are freed promptly when old revisions are replaced

A new test case has been added to verify the behavior: revisions with initialScale > 1 and Reachability = Unreachable can now scale to 0.

Fixes #16260

Proposed Changes

Release Note

Allow unreachable revisions with initialScale > 1 to scale to 0

[knative-prow]

Welcome @aviralgarg05! It looks like this is your first PR to knative/serving 🎉

[knative-prow]

Hi @aviralgarg05. Thanks for your PR.

I'm waiting for a github.com 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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[dprotaso]

/ok-to-test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.14%. Comparing base (09e59ec) to head (acd430d).
⚠️ Report is 27 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16327      +/-   ##
==========================================
+ Coverage   80.11%   80.14%   +0.03%     
==========================================
  Files         215      216       +1     
  Lines       13391    13422      +31     
==========================================
+ Hits        10728    10757      +29     
+ Misses       2304     2298       -6     
- Partials      359      367       +8     

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[dprotaso]

/lgtm
/approve

thanks for the change!

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aviralgarg05, dprotaso

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