Fix memories namespace for Responses API tools (#24898)

## Why

Dedicated memories tools are exposed through a Responses API namespace
tool. The namespace itself has to be a valid tool identifier, so
`memories/` can fail validation before the model ever gets a chance to
call the memory tools.

## What changed

- Changed `MEMORY_TOOLS_NAMESPACE` from `memories/` to `memories`.
- Added `memory_tool_namespace_matches_responses_api_identifier` so the
namespace stays non-empty and limited to Responses-safe identifier
characters.

## Verification

- Added unit coverage for the namespace identifier shape in
`codex-rs/ext/memories/src/tests.rs`.

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

@@ -15,7 +15,7 @@ pub(crate) const MAX_SEARCH_RESULTS: usize = 200;
 pub(crate) const DEFAULT_READ_MAX_TOKENS: usize = 20_000;
 pub(crate) const MEMORY_TOOL_DEVELOPER_INSTRUCTIONS_SUMMARY_TOKEN_LIMIT: usize = 2_500;
 
-pub(crate) const MEMORY_TOOLS_NAMESPACE: &str = "memories/";
+pub(crate) const MEMORY_TOOLS_NAMESPACE: &str = "memories";
 pub(crate) const ADD_AD_HOC_NOTE_TOOL_NAME: &str = "add_ad_hoc_note";
 pub(crate) const LIST_TOOL_NAME: &str = "list";
 pub(crate) const READ_TOOL_NAME: &str = "read";

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

@@ -15,7 +15,7 @@ pub(crate) fn record_tool_call(
         return;
     };
 
-    let tool = format!("{MEMORY_TOOLS_NAMESPACE}{operation}");
+    let tool = format!("{MEMORY_TOOLS_NAMESPACE}/{operation}");
     let _ = metrics_client.counter(
         MEMORIES_TOOL_CALL_METRIC,
         /*inc*/ 1,

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

@@ -22,6 +22,16 @@ use crate::extension::MemoriesExtension;
 use crate::extension::MemoriesExtensionConfig;
 use crate::local::LocalMemoriesBackend;
 
+#[test]
+fn memory_tool_namespace_matches_responses_api_identifier() {
+    assert!(!crate::MEMORY_TOOLS_NAMESPACE.is_empty());
+    assert!(
+        crate::MEMORY_TOOLS_NAMESPACE
+            .bytes()
+            .all(|byte| byte.is_ascii_alphanumeric() || matches!(byte, b'_' | b'-'))
+    );
+}
+
 #[test]
 fn tools_are_not_contributed_without_thread_config() {
     let extension = MemoriesExtension::default();