[codex-analytics] add protocol-native turn timestamps (#16638)

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[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/16638).
* #16870
* #16706
* #16659
* #16641
* #16640
* __->__ #16638

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

@@ -1614,6 +1614,22 @@
     },
     "Turn": {
       "properties": {
+        "completedAt": {
+          "description": "Unix timestamp (in seconds) when the turn completed.",
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "durationMs": {
+          "description": "Duration between turn start and completion in milliseconds, if known.",
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
         "error": {
           "anyOf": [
             {
@@ -1635,6 +1651,14 @@
           },
           "type": "array"
         },
+        "startedAt": {
+          "description": "Unix timestamp (in seconds) when the turn started.",
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
         "status": {
           "$ref": "#/definitions/TurnStatus"
         }