Add model-visible context layout snapshot tests (#12073)

## Summary
- add a dedicated `core/tests/suite/model_visible_layout.rs` snapshot
suite to materialize model-visible request layout in high-value
scenarios
- add three reviewer-focused snapshot scenarios:
  - turn-level context updates (cwd / permissions / personality)
  - first post-resume turn with model hydration + personality change
- first post-resume turn where pre-turn model override matches rollout
model
- wire the new suite into `core/tests/suite/mod.rs`
- commit generated `insta` snapshots under `core/tests/suite/snapshots/`

## Why
This creates a stable, reviewable baseline of model-visible context
layout against `main` before follow-on context-management refactors. It
lets subsequent PRs show focused snapshot diffs for behavior changes
instead of introducing the test surface and behavior changes at once.

## Testing
- `just fmt`
- `INSTA_UPDATE=always cargo test -p codex-core model_visible_layout`

codex-rs/core/tests/suite/mod.rs

@@ -85,6 +85,7 @@ mod model_info_overrides;
 mod model_overrides;
 mod model_switching;
 mod model_tools;
+mod model_visible_layout;
 mod models_cache_ttl;
 mod models_etag_responses;
 mod otel;