Centralize thread environment selection state (#38423)

## What changed

- Make `ThreadEnvironments` the source of truth for live environment selections and use it when building configuration snapshots, permission profiles, per-turn configuration, and MCP refresh inputs.
- Keep settings previews side-effect free while applying accepted environment updates consistently to subsequent turns.
- Move `EnvironmentConfig` into `codex-protocol` so environment ownership no longer depends on a core-only type.

## Testing

- Cover environment previews and snapshots, accepted and rejected steer updates, and active-turn environment stability.

GitOrigin-RevId: 7bcf8fb5df1215f720e4e9c419541a51a871aa5e

codex-rs/core/src/lib.rs

@@ -31,11 +31,11 @@ mod compact_remote;
 mod compact_remote_history;
 mod compact_remote_v2;
 mod compact_token_budget;
+pub use codex_protocol::protocol::EnvironmentConfig;
 pub use codex_thread::BackgroundTerminalInfo;
 pub use codex_thread::CodexThread;
 pub use codex_thread::CodexThreadSettingsOverrides;
 pub use codex_thread::ThreadConfigSnapshot;
-pub use environment_config::EnvironmentConfig;
 pub use session::turn_context::TurnContext;
 mod agent;
 mod agent_communication;
@@ -48,7 +48,6 @@ pub mod context;
 mod context_manager;
 mod current_time;
 mod elicitation;
-mod environment_config;
 mod environment_selection;
 pub mod exec;
 pub mod exec_env;