A75: Add missing E2E test cases for xDS aggregate cluster

[mswierq]

Adds missing E2E test cases for xDS aggregate cluster:

• LRS Load Reporting for Aggregate Cluster:
  • TestAggregateCluster_LRS_PrimaryLeafOnly: Verifies LRS reports stats for the primary leaf cluster when healthy.
  • TestAggregateCluster_LRS_FailoverToSecondaryLeaf: Verifies LRS reports dynamically switch from primary to secondary leaf cluster upon endpoint failover.
  • TestAggregateCluster_LRS_DropsAndLoadReporting: Verifies 100% overload drop category reporting on leaf clusters.
• Telemetry Labels for Aggregate Cluster:
  • TestTelemetryLabels_AggregateCluster: Verifies xDS telemetry labels (grpc.lb.backend_service, grpc.lb.locality) correctly reflect the active leaf cluster and update dynamically during failover.

This PR is part of A75 implementation. However, these test cases work for both the legacy and new LB policy tree.

RELEASE NOTES: n/a

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.18%. Comparing base (89d4d61) to head (304b52f).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9278      +/-   ##
==========================================
+ Coverage   83.14%   83.18%   +0.04%     
==========================================
  Files         422      422              
  Lines       34862    34862              
==========================================
+ Hits        28985    29000      +15     
+ Misses       4385     4374      -11     
+ Partials     1492     1488       -4     

see 26 files with indirect coverage changes

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