Wire metrics client into memories extension (#24567)

## Summary

- let the memories extension capture the process-global OTEL metrics
client at install time
- keep app-server/TUI/exec extension construction APIs unchanged
- store the metrics client for future memory metrics without emitting
any metrics yet

## Test plan

- `just fmt`
- `just bazel-lock-update`
- `just bazel-lock-check`
- Not run: tests/clippy per request; CI will cover them

codex-rs/ext/memories/src/tests.rs

@@ -23,7 +23,7 @@ use crate::local::LocalMemoriesBackend;
 
 #[test]
 fn tools_are_not_contributed_without_thread_config() {
-    let extension = MemoriesExtension;
+    let extension = MemoriesExtension::default();
 
     assert!(
         extension
@@ -37,7 +37,7 @@ fn tools_are_not_contributed_without_thread_config() {
 
 #[test]
 fn tools_are_not_contributed_when_disabled() {
-    let extension = MemoriesExtension;
+    let extension = MemoriesExtension::default();
     let thread_store = ExtensionData::new("thread");
     thread_store.insert(MemoriesExtensionConfig {
         enabled: false,
@@ -53,7 +53,7 @@ fn tools_are_not_contributed_when_disabled() {
 
 #[test]
 fn tools_are_contributed_when_enabled() {
-    let extension = MemoriesExtension;
+    let extension = MemoriesExtension::default();
     let thread_store = ExtensionData::new("thread");
     thread_store.insert(MemoriesExtensionConfig {
         enabled: true,
@@ -111,7 +111,7 @@ async fn prompt_contribution_uses_memory_summary_when_enabled() {
     .await
     .expect("write memory summary");
 
-    let extension = MemoriesExtension;
+    let extension = MemoriesExtension::default();
     let thread_store = ExtensionData::new("thread");
     thread_store.insert(MemoriesExtensionConfig {
         enabled: true,