Preserve annotations when omitting unsupported media (#40277)

## What changed

- Render omitted image and audio inputs as contextual fragments tagged with
  `images.unsupported` and `audio.unsupported` content kinds.
- Normalize message media through annotated content so metadata for surrounding
  content remains intact when switching to a model without media support.

## Testing

- Extend history and model-switching tests to verify content-kind annotations
  before and after unsupported media is replaced.

GitOrigin-RevId: 4ee6577f8b78bd5a850ee01ea9d4590c20272850

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

@@ -38,6 +38,7 @@ mod rollout_budget;
 mod subagent_notification;
 mod token_budget_context;
 mod turn_aborted;
+mod unsupported_media;
 mod user_instructions;
 mod user_shell_command;
 pub(crate) mod world_state;
@@ -98,6 +99,7 @@ pub(crate) use token_budget_context::TokenBudgetContext;
 pub(crate) use token_budget_context::TokenBudgetRemainingContext;
 pub(crate) use token_budget_context::TokenBudgetReminder;
 pub(crate) use turn_aborted::TurnAborted;
+pub(crate) use unsupported_media::UnsupportedMedia;
 pub(crate) use user_instructions::UserInstructions;
 pub(crate) use user_shell_command::UserShellCommand;
 pub(crate) use world_state::ManagedDeveloperInstructions;

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

@@ -0,0 +1,42 @@
+use super::ContextualUserFragment;
+use codex_protocol::models::ContentItemKind;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct UnsupportedMedia {
+    text: &'static str,
+    content_kind: &'static str,
+}
+
+impl UnsupportedMedia {
+    pub(crate) const IMAGE: Self = Self {
+        text: "image content omitted because you do not support image input",
+        content_kind: "images.unsupported",
+    };
+
+    pub(crate) const AUDIO: Self = Self {
+        text: "audio content omitted because you do not support audio input",
+        content_kind: "audio.unsupported",
+    };
+}
+
+impl ContextualUserFragment for UnsupportedMedia {
+    fn content_kind(&self) -> ContentItemKind {
+        ContentItemKind(self.content_kind.to_string())
+    }
+
+    fn role(&self) -> &'static str {
+        "user"
+    }
+
+    fn markers(&self) -> (&'static str, &'static str) {
+        Self::type_markers()
+    }
+
+    fn type_markers() -> (&'static str, &'static str) {
+        ("", "")
+    }
+
+    fn body(&self) -> String {
+        self.text.to_string()
+    }
+}