add tag-to-host support

[kahirokunn]

Summary

• Implement networking.internal.knative.dev/tag-to-host annotation support for Gateway API HTTPRoutes
• When a KIngress carries the tag-to-host annotation, the reconciler synthesizes additional host-based HTTPRoutes from header-based tag paths
• This enables ingress implementations that cannot do header-based routing to route by hostname instead

Test plan

• Unit tests for DesiredHTTPRouteRules with tag-to-host annotation
• Unit tests for deduplication (existing host-based rules not duplicated)
• Integration test for reconciler with tag-to-host annotation
• All existing tests pass

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: kahirokunn
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[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.33%. Comparing base (13e2037) to head (2edaa5a).

Files with missing lines	Patch %	Lines
pkg/reconciler/ingress/resources/tags.go	71.42%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #944      +/-   ##
==========================================
- Coverage   76.38%   76.33%   -0.05%     
==========================================
  Files          17       18       +1     
  Lines        1266     1289      +23     
==========================================
+ Hits          967      984      +17     
- Misses        263      268       +5     
- Partials       36       37       +1     

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