[codex] Add comp_hash to model metadata (#27532)

## Summary
- add optional `comp_hash` metadata to `ModelInfo`
- update `ModelInfo` fixtures for the shared schema change
- keep older model responses compatible by defaulting the field to
`None`

## Why
The models endpoint needs an opaque identifier for compaction-compatible
model configurations. This PR only exposes that value in model metadata;
it does not add it to turn context or change runtime behavior.

Follow-up #27520 carries the value through turn context and rollouts,
then uses it to trigger compaction.

## Stack
- based directly on `main`
- replaces #27519, which was accidentally merged into the wrong base
branch
- functionality follow-up: #27520

## Testing
- `just test -p codex-protocol
model_info_defaults_availability_nux_to_none_when_omitted`
- `just fix -p codex-core -p codex-protocol -p codex-analytics -p
codex-models-manager`

codex-rs/protocol/src/openai_models.rs

@@ -389,6 +389,9 @@ pub struct ModelInfo {
     /// context window when available.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub auto_compact_token_limit: Option<i64>,
+    /// Opaque identifier for compaction-compatible model configurations.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub comp_hash: Option<String>,
     /// Percentage of the context window considered usable for inputs, after
     /// reserving headroom for system prompts, tool overhead, and model output.
     #[serde(default = "default_effective_context_window_percent")]
@@ -671,6 +674,7 @@ mod tests {
             context_window: None,
             max_context_window: None,
             auto_compact_token_limit: None,
+            comp_hash: None,
             effective_context_window_percent: 95,
             experimental_supported_tools: vec![],
             input_modalities: default_input_modalities(),
@@ -940,6 +944,7 @@ mod tests {
         assert_eq!(model.web_search_tool_type, WebSearchToolType::Text);
         assert!(!model.supports_search_tool);
         assert!(!model.use_responses_lite);
+        assert_eq!(model.comp_hash, None);
         assert_eq!(model.auto_review_model_override, None);
         assert_eq!(model.tool_mode, None);
     }