Prevent cross-service revision traffic routing

[linkvt]

Fixes #11916

Services can no longer route traffic to revisions belonging to different services. When a Service-owned Route references a revision with a different serving.knative.dev/service label, the Route will show Ready=False with reason RevisionNotOwned.

Standalone Routes (not owned by a Service) are unaffected and can still reference any revision.

Changes

• Validate revision ownership when building traffic configuration
• New RevisionNotOwned condition reason when validation fails
• Standalone Routes (not owned by a Service) are unaffected

Open Question

Should we also add validation at the webhook level for faster user feedback?

This would require using the revision informer in the webhook which would probably increase the memory footprint a bit (didn't test it).
The current implementation validates at reconciliation time only, which is consistent with how other traffic errors (e.g., RevisionMissing) are handled so I'm fine keeping the implementation like this.

Release Note

Services can no longer route traffic to revisions belonging to different services; attempting to do so will result in Ready=False with reason RevisionNotOwned.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.12%. Comparing base (5fbd94e) to head (3ab7621).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/reconciler/route/traffic/errors.go	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16294      +/-   ##
==========================================
+ Coverage   80.10%   80.12%   +0.02%     
==========================================
  Files         215      215              
  Lines       13332    13360      +28     
==========================================
+ Hits        10679    10705      +26     
- Misses       2294     2296       +2     
  Partials      359      359              

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[linkvt]

/retest

[linkvt]

/retest

[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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