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")