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/tui/src/chatwidget/tests/helpers.rs

@@ -283,7 +283,6 @@ fn test_model_info(slug: &str, priority: i32, supports_fast_mode: bool) -> Model
         "default_service_tier": null,
         "availability_nux": null,
         "upgrade": null,
-        "base_instructions": "base instructions",
         "default_reasoning_summary": "none",
         "support_verbosity": false,
         "default_verbosity": null,