Classify internal model context by source (#40294)

## What changed

- Derive each `InternalModelContextFragment` content kind from its source as
  `<source>.internal_context` instead of using a shared generic kind.
- Cover the source-specific kind in the contextual user fragment test.

GitOrigin-RevId: 2075d558265fc1ae1ce394e9a865210d1bc270af

codex-rs/core/src/context/contextual_user_message_tests.rs

@@ -5,6 +5,7 @@ use crate::context::InternalModelContextFragment;
 use crate::context::SubagentNotification;
 use codex_protocol::items::HookPromptFragment;
 use codex_protocol::items::build_hook_prompt_message;
+use codex_protocol::models::ContentItemKind;
 use codex_protocol::models::ResponseItem;
 use pretty_assertions::assert_eq;
 
@@ -72,12 +73,16 @@ fn detects_subagent_notification_fragment_case_insensitively() {
 
 #[test]
 fn detects_internal_model_context_fragment() {
-    let text = InternalModelContextFragment::new(
+    let fragment = InternalModelContextFragment::new(
         InternalContextSource::from_static("extension"),
         "Internal steering.",
-    )
-    .render();
+    );
+    let text = fragment.render();
 
+    assert_eq!(
+        fragment.content_kind(),
+        ContentItemKind("extension.internal_context".to_string())
+    );
     assert_eq!(
         text,
         "<codex_internal_context source=\"extension\">\nInternal steering.\n</codex_internal_context>"

codex-rs/core/src/context/internal_model_context.rs

@@ -78,7 +78,7 @@ impl InternalModelContextFragment {
 
 impl ContextualUserFragment for InternalModelContextFragment {
     fn content_kind(&self) -> ContentItemKind {
-        ContentItemKind("generic.internal_model_context".to_string())
+        ContentItemKind(format!("{}.internal_context", self.source.as_str()))
     }
 
     fn role(&self) -> &'static str {