Add sticky environment API and thread state (#18897)

## Summary
- add sticky environment selections to app-server v2 thread/start and
turn/start request flow
- carry thread-level selections through core session/thread state
- add app-server coverage for sticky selections and turn overrides

## Stack
1. This PR: API and thread persistence
2. #18898: config.toml named environment loading
3. #18899: downstream tool/runtime consumers

## Validation
- Not run locally; split only.

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Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/protocol.rs

@@ -432,7 +432,7 @@ pub enum Op {
     UserInput {
         /// User input items, see `InputItem`
         items: Vec<UserInput>,
-        /// Optional turn-scoped environment selections.
+        /// Optional turn-scoped environments.
         #[serde(default, skip_serializing_if = "Option::is_none")]
         environments: Option<Vec<TurnEnvironmentSelection>>,
         /// Optional JSON Schema used to constrain the final assistant message for this turn.
@@ -579,7 +579,7 @@ pub enum Op {
         #[serde(skip_serializing_if = "Option::is_none")]
         personality: Option<Personality>,
 
-        /// Optional turn-scoped environment selections.
+        /// Optional turn-scoped environments.
         #[serde(default, skip_serializing_if = "Option::is_none")]
         environments: Option<Vec<TurnEnvironmentSelection>>,
     },