Add app-server environment registry for exec-server routing

Expose app-server environment register/list RPCs, persist environment selection through thread lifecycle flows, and cover the new routing behavior with app-server tests.

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

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

@@ -1029,6 +1029,13 @@
         "null"
       ]
     },
+    "environmentId": {
+      "description": "Override the execution environment used when resuming this thread.",
+      "type": [
+        "string",
+        "null"
+      ]
+    },
     "model": {
       "description": "Configuration overrides for the resumed thread, if any.",
       "type": [