fix bug with grpcroute mirror filter

[aburanrbx]

What type of PR is this?
fix(provider): add RequestMirror backendRef indexing for GRPCRoute

What this PR does / why we need it:

The GRPCRoute backend indexer (backendGRPCRouteIndexFunc) is missing RequestMirror filter support. It only iterates over rule.BackendRefs and never inspects rule.Filters for RequestMirror entries. Since the indexer tells the resource collector which services a route depends on, the mirror target service is never loaded into the translation resource map. When validateBackendRefService calls t.GetService(), the service isn't found and the route is rejected with "service not found" — even though the service exists in the cluster.

This is the same bug as #5495 (for HTTPRoute), which was fixed in #5497, but the fix was only applied to backendHTTPRouteIndexFunc and never carried over to backendGRPCRouteIndexFunc.

The fix adds the same RequestMirror filter iteration to backendGRPCRouteIndexFunc that already exists in backendHTTPRouteIndexFunc, appending the mirror's BackendRef to the indexed backend refs.

Which issue(s) this PR fixes:

Fixes #6875

Release Notes: Yes

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[zirain]

can you add test and release notes for this?

[arkodg]

LGTM thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.18%. Comparing base (dd0c09f) to head (f8b1081).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8541      +/-   ##
==========================================
+ Coverage   74.16%   74.18%   +0.01%     
==========================================
  Files         242      242              
  Lines       37804    37821      +17     
==========================================
+ Hits        28039    28056      +17     
- Misses       7809     7810       +1     
+ Partials     1956     1955       -1     

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