feat(app-server): add tracing to all app-server APIs (#13285)

### Overview
This PR adds the first piece of tracing for app-server JSON-RPC
requests.

There are two main changes:
- JSON-RPC requests can now take an optional W3C trace context at the
top level via a `trace` field (`traceparent` / `tracestate`).
- app-server now creates a dedicated request span for every inbound
JSON-RPC request in `MessageProcessor`, and uses the request-level trace
context as the parent when present.

For compatibility with existing flows, app-server still falls back to
the TRACEPARENT env var when there is no request-level traceparent.

This PR is intentionally scoped to the app-server boundary. In a
followup, we'll actually propagate trace context through the async
handoff into core execution spans like run_turn, which will make
app-server traces much more useful.

### Spans
A few details on the app-server span shape:
- each inbound request gets its own server span
- span/resource names are based on the JSON-RPC method (`initialize`,
`thread/start`, `turn/start`, etc.)
- spans record transport (stdio vs websocket), request id, connection
id, and client name/version when available
- `initialize` stores client metadata in session state so later requests
on the same connection can reuse it

codex-rs/app-server-protocol/schema/json/JSONRPCRequest.json

@@ -11,6 +11,23 @@
           "type": "integer"
         }
       ]
+    },
+    "W3cTraceContext": {
+      "properties": {
+        "traceparent": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "tracestate": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "type": "object"
     }
   },
   "description": "A request that expects a response.",
@@ -21,7 +38,18 @@
     "method": {
       "type": "string"
     },
-    "params": true
+    "params": true,
+    "trace": {
+      "anyOf": [
+        {
+          "$ref": "#/definitions/W3cTraceContext"
+        },
+        {
+          "type": "null"
+        }
+      ],
+      "description": "Optional W3C Trace Context for distributed tracing."
+    }
   },
   "required": [
     "id",