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/codex_app_server_protocol.schemas.json

@@ -17253,6 +17253,16 @@
                 "type": "null"
               }
             ]
+          },
+          "usageMetadata": {
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/ResponseUsageMetadata"
+              },
+              {
+                "type": "null"
+              }
+            ]
           }
         },
         "required": [
@@ -18501,6 +18511,18 @@
           }
         ]
       },
+      "ResponseUsageMetadata": {
+        "description": "Usage metadata reported for one upstream response.",
+        "properties": {
+          "amount": {
+            "type": [
+              "string",
+              "null"
+            ]
+          }
+        },
+        "type": "object"
+      },
       "ResponsesApiWebSearchAction": {
         "oneOf": [
           {