Fix compilation errors and test failures across tpu_sync targets

- Qualify PollSaveStatusResult and include kv_cache_store.h in
  kv_cache_store_mock.h/cc, adding missing kv_cache dependency.
- Rename mock registration macro in kv_cache_store_backend_mock.h to
  REGISTER_MOCK_KV_CACHE_STORE_BACKEND to prevent macro collision with
  the production kv_cache_store_backend_factory.h macro.
- Add torch_tpu_device_buffer_headers dependency to
  weight_synchronizer_torch_mock to fix layering check failure.
- Update PollSaveStatus() structured bindings in storage_offload_test.cc
  to unpack all 5 fields of PollSaveStatusResult.
- Add default metric instance registration in RegisterKnownFamilies() in
  prometheus_exporter.cc to ensure text snapshot serializes registered
  metric families on initialization.
- Configure pathways_qwen_weight_synchronization_perf_test with reduced
  layers/iterations and appropriate perf/hardware test tags (notap,
  cpu:8, requires-accel-hbm, requires-net:external).

Tested:
- blaze test --nocheck_visibility //third_party/tpu_raiden/tpu_sync/experimental/feat_storage_extension:kv_cache_store_mock
- blaze test --nocheck_visibility //third_party/tpu_raiden/tpu_sync/experimental/feat_storage_extension:storage_offload_test
- blaze test --nocheck_visibility //third_party/tpu_raiden/tpu_sync/frameworks/torch:weight_synchronizer_torch_mock
- blaze test --nocheck_visibility //third_party/tpu_raiden/tpu_sync/telemetry/python:telemetry_binding_test
- blaze test --nocheck_visibility //third_party/tpu_raiden/tpu_sync/telemetry:prometheus_exporter_test
- Full test pass across //third_party/tpu_raiden/tpu_sync/...
PiperOrigin-RevId: 969954875

Author: justinlu13

tpu_sync/frameworks/torch/BUILD

@@ -429,6 +429,7 @@ cc_library(
     tags = ["nobuilder"],
     visibility = ["//visibility:public"],
     deps = [
+        ":torch_tpu_device_buffer_headers",
         ":torch_tpu_tensor_to_buffer_headers",
         ":torch_utils_mock",
         "//tpu_sync/weight_sync:weight_synchronizer_base",

tpu_sync/telemetry/prometheus_exporter.cc

@@ -75,6 +75,7 @@ void PrometheusExporter::RegisterKnownFamilies() {
                             .Name(prometheus_name)
                             .Help(std::string(meta.description))
                             .Register(*registry_);
+        family->Add({});
         counter_families_.emplace(meta.name, family);
         break;
       }
@@ -83,6 +84,7 @@ void PrometheusExporter::RegisterKnownFamilies() {
                             .Name(prometheus_name)
                             .Help(std::string(meta.description))
                             .Register(*registry_);
+        family->Add({});
         gauge_families_.emplace(meta.name, family);
         break;
       }
@@ -91,6 +93,7 @@ void PrometheusExporter::RegisterKnownFamilies() {
                             .Name(prometheus_name)
                             .Help(std::string(meta.description))
                             .Register(*registry_);
+        family->Add({}, default_buckets_);
         histogram_families_.emplace(meta.name, family);
         break;
       }