Implement protocol-aware gRPC health checks

[kacy]

Summary:

• replace the transport-only gRPC probe with a real grpc.health.v1.Health/Check request over HTTP/2
• add optional health_check.service manifest support for named gRPC services
• update the gRPC health-check docs to describe the protocol-level behavior

Validation:

• env YOQ_SKIP_SLOW_TESTS=1 ZIG_GLOBAL_CACHE_DIR=.zig-global-cache ZIG_LOCAL_CACHE_DIR=.zig-local-cache zig build test
• 1765 passed; 26 skipped; 0 failed