Wire A32 circuit breaking into xDS channels

[mingley]

Summary

• Wire gRFC A32 max_requests into xDS endpoint dispatch with process-global {cluster, EDS service} counters and response-lifetime permits (01c865f8, 36b03763).
• Isolate CDS/EDS removal and re-add generations so stale limits, connectors, and endpoints are not reused (53fe418a).
• Return local limit drops as non-retryable UNAVAILABLE and retain the A32 default of 1024 when CDS omits max_requests (c6b89daa).

Testing Done

• cargo fmt --all --check
• cargo test -p tonic-xds --all-features --quiet — 415 tests and 4 doctests passed
• cargo clippy -p tonic-xds --all-features --all-targets --quiet
• python3 tools/check_license.py — 409 files passed

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mingley / name: Michael Ingley (4609832, 7505002)