Wire A32 circuit breaking into xDS channels

[mingley]

Motivation

This is the production wiring follow-up to #2711. The limiter exists after #2711, but tonic-xds still needs to read CDS circuit-breaking config and place enforcement in the request path before A32 support is active.

This PR is intentionally draft until #2711 lands. It is currently based on master because #2711's branch lives in a fork; after #2711 lands, this branch should be rebased so the diff contains only the wiring commit.

Solution

• Carry parsed A32 CircuitBreakingConfig on validated ClusterResource values.
• Watch CDS updates from the circuit-breaking layer and wait for CDS before acquiring permits to avoid startup races.
• Insert circuit breaking between routing and retry in XdsChannelBuilder, so circuit-broken calls do not enter retry policy.
• Add channel-level coverage for injected limits, CDS-driven limits, and waiting for CDS before enforcement.

Testing Done

• cargo fmt --all --check
• cargo check -p tonic-xds --all-features
• cargo test -p tonic-xds --lib

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

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