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/EnvironmentRegisterParams.json

@@ -0,0 +1,19 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "environmentId": {
+      "type": "string"
+    },
+    "execServerUrl": {
+      "type": [
+        "string",
+        "null"
+      ]
+    }
+  },
+  "required": [
+    "environmentId"
+  ],
+  "title": "EnvironmentRegisterParams",
+  "type": "object"
+}