feat: retain NetworkProxy, when appropriate (#11207)

As of this PR, `SessionServices` retains a
`Option<StartedNetworkProxy>`, if appropriate.

Now the `network` field on `Config` is `Option<NetworkProxySpec>`
instead of `Option<NetworkProxy>`.

Over in `Session::new()`, we invoke `NetworkProxySpec::start_proxy()` to
create the `StartedNetworkProxy`, which is a new struct that retains the
`NetworkProxy` as well as the `NetworkProxyHandle`. (Note that `Drop` is
implemented for `NetworkProxyHandle` to ensure the proxies are shutdown
when it is dropped.)

The `NetworkProxy` from the `StartedNetworkProxy` is threaded through to
the appropriate places.


---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/11207).
* #11285
* __->__ #11207

codex-rs/app-server-protocol/schema/json/v1/ResumeConversationResponse.json

@@ -887,6 +887,17 @@
             "model_provider_id": {
               "type": "string"
             },
+            "network_proxy": {
+              "anyOf": [
+                {
+                  "$ref": "#/definitions/SessionNetworkProxyRuntime"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "description": "Runtime proxy bind addresses, when the managed proxy was started for this session."
+            },
             "reasoning_effort": {
               "anyOf": [
                 {
@@ -4427,6 +4438,25 @@
         }
       ]
     },
+    "SessionNetworkProxyRuntime": {
+      "properties": {
+        "admin_addr": {
+          "type": "string"
+        },
+        "http_addr": {
+          "type": "string"
+        },
+        "socks_addr": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "admin_addr",
+        "http_addr",
+        "socks_addr"
+      ],
+      "type": "object"
+    },
     "SkillDependencies": {
       "properties": {
         "tools": {
@@ -5186,4 +5216,4 @@
   ],
   "title": "ResumeConversationResponse",
   "type": "object"
-}
+}