[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/core/src/tools/code_mode/mod.rs

@@ -30,7 +30,7 @@ use crate::tools::parallel::ToolCallRuntime;
 use crate::tools::router::ToolCall;
 use crate::tools::router::ToolCallSource;
 use crate::unified_exec::resolve_max_tokens;
-use codex_features::Feature;
+use codex_protocol::openai_models::ToolMode;
 use codex_tools::ToolName;
 use codex_utils_output_truncation::TruncationPolicy;
 use codex_utils_output_truncation::formatted_truncate_text_content_items_with_policy;
@@ -91,7 +91,7 @@ impl CodeModeService {
         router: Arc<ToolRouter>,
         tracker: SharedTurnDiffTracker,
     ) -> Option<codex_code_mode::CodeModeTurnWorker> {
-        if !turn.features.enabled(Feature::CodeMode) {
+        if !matches!(turn.tool_mode, ToolMode::CodeMode | ToolMode::CodeModeOnly) {
             return None;
         }