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/app-server/tests/common/models_cache.rs

@@ -43,6 +43,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         supports_parallel_tool_calls: false,
         supports_image_detail_original: false,
         context_window: Some(272_000),
+        max_context_window: None,
         auto_compact_token_limit: None,
         effective_context_window_percent: 95,
         experimental_supported_tools: Vec::new(),