Expose response usage metadata in completion events (#41087)

## What changed

- Parse optional `usage_metadata.amount` values from Responses API completion events without converting their string representation.
- Propagate the metadata through SSE and WebSocket streams, regular turns, and remote compaction.
- Add `usageMetadata` to the app-server `rawResponse/completed` notification and its generated schemas.

## Testing

- Cover missing, null, zero, and high-precision amounts, including completion events emitted during compaction.

GitOrigin-RevId: b16d81e8350eab0dc7ff613d0dc9dbed73041713

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

@@ -1,6 +1,18 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "definitions": {
+    "ResponseUsageMetadata": {
+      "description": "Usage metadata reported for one upstream response.",
+      "properties": {
+        "amount": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "type": "object"
+    },
     "TokenUsageBreakdown": {
       "properties": {
         "cacheWriteInputTokens": {
@@ -59,6 +71,16 @@
           "type": "null"
         }
       ]
+    },
+    "usageMetadata": {
+      "anyOf": [
+        {
+          "$ref": "#/definitions/ResponseUsageMetadata"
+        },
+        {
+          "type": "null"
+        }
+      ]
     }
   },
   "required": [