Add max context window model metadata (#18382)

Adds max_context_window to model metadata and routes core context-window
reads through resolved model info. Config model_context_window overrides
are clamped to max_context_window when present; without an override, the
model context_window is used.

codex-rs/models-manager/src/manager_tests.rs

@@ -70,6 +70,7 @@ fn remote_model_with_visibility(
             "supports_parallel_tool_calls": false,
             "supports_image_detail_original": false,
             "context_window": 272_000,
+            "max_context_window": 272_000,
             "experimental_supported_tools": [],
         }))
         .expect("valid model")

codex-rs/models-manager/src/model_info.rs

@@ -27,7 +27,13 @@ pub fn with_config_overrides(mut model: ModelInfo, config: &ModelsManagerConfig)
         model.supports_reasoning_summaries = true;
     }
     if let Some(context_window) = config.model_context_window {
-        model.context_window = Some(context_window);
+        model.context_window = Some(
+            model
+                .max_context_window
+                .map_or(context_window, |max_context_window| {
+                    context_window.min(max_context_window)
+                }),
+        );
     }
     if let Some(auto_compact_token_limit) = config.model_auto_compact_token_limit {
         model.auto_compact_token_limit = Some(auto_compact_token_limit);
@@ -84,6 +90,7 @@ pub fn model_info_from_slug(slug: &str) -> ModelInfo {
         supports_parallel_tool_calls: false,
         supports_image_detail_original: false,
         context_window: Some(272_000),
+        max_context_window: Some(272_000),
         auto_compact_token_limit: None,
         effective_context_window_percent: 95,
         experimental_supported_tools: Vec::new(),

codex-rs/models-manager/src/model_info_tests.rs

@@ -43,3 +43,32 @@ fn reasoning_summaries_override_false_is_noop_when_model_is_false() {
 
     assert_eq!(updated, model);
 }
+
+#[test]
+fn model_context_window_override_clamps_to_max_context_window() {
+    let mut model = model_info_from_slug("unknown-model");
+    model.context_window = Some(273_000);
+    model.max_context_window = Some(400_000);
+    let config = ModelsManagerConfig {
+        model_context_window: Some(500_000),
+        ..Default::default()
+    };
+
+    let updated = with_config_overrides(model.clone(), &config);
+    let mut expected = model;
+    expected.context_window = Some(400_000);
+
+    assert_eq!(updated, expected);
+}
+
+#[test]
+fn model_context_window_uses_model_value_without_override() {
+    let mut model = model_info_from_slug("unknown-model");
+    model.context_window = Some(273_000);
+    model.max_context_window = Some(400_000);
+    let config = ModelsManagerConfig::default();
+
+    let updated = with_config_overrides(model.clone(), &config);
+
+    assert_eq!(updated, model);
+}