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.

---------

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

codex-rs/app-server-protocol/schema/json/v2/ThreadStartParams.json

@@ -388,6 +388,21 @@
         "clear"
       ],
       "type": "string"
+    },
+    "TurnEnvironmentParams": {
+      "properties": {
+        "cwd": {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        },
+        "environmentId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "cwd",
+        "environmentId"
+      ],
+      "type": "object"
     }
   },
   "properties": {