[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/codex-api/tests/models_integration.rs

@@ -93,6 +93,7 @@ async fn models_client_hits_models_endpoint() {
             context_window: Some(272_000),
             max_context_window: None,
             auto_compact_token_limit: None,
+            comp_hash: None,
             effective_context_window_percent: 95,
             experimental_supported_tools: Vec::new(),
             input_modalities: default_input_modalities(),