[codex] Add model tool mode selector (#25031)

## Why
Some models need to select their code-execution behavior through model
catalog metadata. Models without that metadata must continue to follow
the existing `CodeMode` and `CodeModeOnly` feature flags, including when
a newer server sends an enum value this client does not recognize.

## What changed
- add optional `ModelInfo.tool_mode` metadata with `direct`,
`code_mode`, and `code_mode_only`
- treat omitted and unknown wire values as `None`
- resolve `None` from the existing feature flags
- carry the resolved `ToolMode` directly on `TurnContext`, outside
`Config`
- use the resolved value for turn creation, model switches, review
turns, tool planning, and code execution

## Coverage
- add protocol coverage for omitted, known, and unknown enum values
- add focused coverage for flag fallback and explicit metadata
overriding feature flags
- add core integration coverage that fetches remote model metadata
through `/v1/models` and verifies the outbound `/responses` tools for
explicit `direct` and `code_mode_only` selectors

## Stack
- followed by #25032

codex-rs/codex-api/tests/models_integration.rs

@@ -98,6 +98,7 @@ async fn models_client_hits_models_endpoint() {
             input_modalities: default_input_modalities(),
             used_fallback_model_metadata: false,
             supports_search_tool: false,
+            tool_mode: None,
         }],
     };