Propagate safety buffering events to app-server clients (#29371)

Responses API safety buffering metadata currently stops at the transport
boundary, so app-server clients cannot render the in-progress safety
review state.

This change:
- decodes and deduplicates `safety_buffering` metadata from Responses
API SSE and WebSocket events without suppressing the original response
event
- emits a typed core event containing the requested model plus backend
use cases and reasons
- forwards that event as `turn/safetyBuffering/updated` through
app-server v2 and updates generated protocol schemas
- keeps the side-channel event out of persisted rollouts and turn timing

This supports the Codex Apps buffering UX and depends on the Responses
API backend work in https://github.com/openai/openai/pull/1044569 and
https://github.com/openai/openai/pull/1044571.

Validation:
- focused `codex-core` safety-buffering integration test passes
- `cargo check -p codex-core -p codex-app-server -p
codex-app-server-protocol`
- `just fix -p codex-api -p codex-protocol -p codex-core -p
codex-app-server-protocol -p codex-app-server -p codex-rollout -p
codex-rollout-trace -p codex-otel`
- `just fmt`
- broad package test run: 4,430/4,492 passed; 62 unrelated
local-environment/concurrency failures involved unavailable test
binaries, MCP subprocess setup, and app-server timeouts

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

@@ -2583,6 +2583,39 @@
       ],
       "type": "object"
     },
+    "ModelSafetyBufferingUpdatedNotification": {
+      "properties": {
+        "model": {
+          "type": "string"
+        },
+        "reasons": {
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
+        },
+        "threadId": {
+          "type": "string"
+        },
+        "turnId": {
+          "type": "string"
+        },
+        "useCases": {
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
+        }
+      },
+      "required": [
+        "model",
+        "reasons",
+        "threadId",
+        "turnId",
+        "useCases"
+      ],
+      "type": "object"
+    },
     "ModelVerification": {
       "enum": [
         "trustedAccessForCyber"
@@ -6608,6 +6641,26 @@
       "title": "Turn/moderationMetadataNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "model/safetyBuffering/updated"
+          ],
+          "title": "Model/safetyBuffering/updatedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ModelSafetyBufferingUpdatedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Model/safetyBuffering/updatedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {