Support model-owned token budget defaults (#35608)

## What changed

- Add token-budget settings to model catalog messages and apply them when the
  feature is enabled without explicit token-budget configuration.
- Keep explicit user settings authoritative, reject invalid catalog defaults,
  and preserve resolved defaults in exported config locks.
- Manage context-window guidance through world state so it updates once when
  the active model changes while retaining prior conversation history.

## Testing

- Cover catalog defaults, explicit overrides, disabled features, invalid
  values, config-lock replay, and model switching.

GitOrigin-RevId: 54544fefaa14b09f5ef5bad9967a13c52b87b0fe

codex-rs/core/src/context/world_state/context_window_guidance.rs

@@ -0,0 +1,54 @@
+use super::PreviousSectionState;
+use super::WorldStateSection;
+use crate::context::ContextWindowGuidance;
+use crate::context::ContextualUserFragment;
+
+/// Model-visible guidance for managing the current context window.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(crate) struct ContextWindowGuidanceState {
+    message: String,
+}
+
+impl ContextWindowGuidanceState {
+    pub(crate) fn new(message: &str) -> Self {
+        Self {
+            message: message.to_string(),
+        }
+    }
+}
+
+impl WorldStateSection for ContextWindowGuidanceState {
+    const ID: &'static str = "context_window_guidance";
+    type Snapshot = String;
+
+    fn snapshot(&self) -> Self::Snapshot {
+        self.message.clone()
+    }
+
+    fn matches_legacy_fragment(role: &str, text: &str) -> bool {
+        role == "developer" && ContextWindowGuidance::matches_text(text)
+    }
+
+    fn has_retained_fragment_matcher() -> bool {
+        true
+    }
+
+    fn matches_retained_fragment(role: &str, text: &str) -> bool {
+        Self::matches_legacy_fragment(role, text)
+    }
+
+    fn render_diff(
+        &self,
+        previous: PreviousSectionState<'_, Self::Snapshot>,
+    ) -> Option<Box<dyn ContextualUserFragment>> {
+        if matches!(previous, PreviousSectionState::Known(message) if message == &self.message) {
+            return None;
+        }
+
+        Some(Box::new(ContextWindowGuidance::new(&self.message)))
+    }
+}
+
+#[cfg(test)]
+#[path = "context_window_guidance_tests.rs"]
+mod tests;

codex-rs/core/src/context/world_state/context_window_guidance_tests.rs

@@ -0,0 +1,52 @@
+use super::ContextWindowGuidanceState;
+use crate::context::ContextWindowGuidance;
+use crate::context::ContextualUserFragment;
+use crate::context::world_state::WorldState;
+use codex_protocol::models::ResponseItem;
+use pretty_assertions::assert_eq;
+
+#[test]
+fn changed_guidance_is_rendered_once() {
+    let mut original = WorldState::default();
+    original.add_section(ContextWindowGuidanceState::new("original guidance"));
+    let snapshot = original.snapshot();
+
+    let mut unchanged = WorldState::default();
+    unchanged.add_section(ContextWindowGuidanceState::new("original guidance"));
+    assert!(unchanged.render_diff(&snapshot).is_empty());
+
+    let mut refreshed = WorldState::default();
+    refreshed.add_section(ContextWindowGuidanceState::new("refreshed guidance"));
+    let fragments = refreshed.render_diff(&snapshot);
+
+    assert_eq!(fragments.len(), 1);
+    assert_eq!(
+        fragments[0].render(),
+        ContextWindowGuidance::new("refreshed guidance").render()
+    );
+}
+
+#[test]
+fn legacy_guidance_is_reconciled_once() {
+    let mut state = WorldState::default();
+    state.add_section(ContextWindowGuidanceState::new("current guidance"));
+    let retained: ResponseItem =
+        ContextualUserFragment::into(ContextWindowGuidance::new("previous guidance"));
+
+    let fragments =
+        state.render_history_diff(/*previous*/ None, std::slice::from_ref(&retained));
+    assert_eq!(fragments.len(), 1);
+    assert_eq!(
+        fragments[0].render(),
+        ContextWindowGuidance::new("current guidance").render()
+    );
+
+    let snapshot = state.snapshot();
+    let reconciled: ResponseItem =
+        ContextualUserFragment::into(ContextWindowGuidance::new("current guidance"));
+    assert!(
+        state
+            .render_history_diff(Some(&snapshot), &[retained, reconciled])
+            .is_empty()
+    );
+}

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

@@ -1,6 +1,7 @@
 mod agents_md;
 mod apps_instructions;
 mod collaboration_mode;
+mod context_window_guidance;
 mod environment;
 mod environments_instructions;
 mod model;
@@ -32,6 +33,7 @@ use std::fmt;
 pub(crate) use agents_md::AgentsMdState;
 pub(crate) use apps_instructions::AppsInstructionsState;
 pub(crate) use collaboration_mode::CollaborationModeState;
+pub(crate) use context_window_guidance::ContextWindowGuidanceState;
 pub(crate) use environment::EnvironmentsState;
 pub(crate) use environments_instructions::EnvironmentsInstructionsState;
 pub(crate) use model::ModelInstructionsState;