Consolidate model instructions in ModelMessages (#36787)

## What changed

- Remove `ModelInfo.base_instructions` as an in-memory instruction source and use `model_messages.instructions_template` consistently for bundled, remote, fallback, and overridden model metadata.
- Preserve compatibility by promoting legacy `base_instructions` values when reading model responses and caches, and by including rendered legacy instructions when serializing `ModelsResponse` for older clients.
- Treat templates without instruction variables as literal text and retain the other model-message fields when applying instruction overrides.

This completes the consolidation proposed in https://github.com/openai/codex/pull/31302.

## Testing

- Cover legacy response and cache migration, canonical-template precedence, fallback instructions, personality rendering, overrides, and model switching.

GitOrigin-RevId: 089d986ca5e30da67db2c77a1b6a046d2cff52dc

codex-rs/codex-api/src/endpoint/models.rs

@@ -208,7 +208,6 @@ mod tests {
                     "supported_in_api": true,
                     "priority": 1,
                     "upgrade": null,
-                    "base_instructions": "base instructions",
                     "support_verbosity": false,
                     "default_verbosity": null,
                     "apply_patch_tool_type": null,

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

@@ -79,7 +79,6 @@ async fn models_client_hits_models_endpoint() {
             service_tiers: Vec::new(),
             default_service_tier: None,
             upgrade: None,
-            base_instructions: "base instructions".to_string(),
             model_messages: None,
             include_skills_usage_instructions: false,
             supports_reasoning_summary_parameter: true,