app-server service tier plumbing (plus some cleanup) (#13334)

followup to https://github.com/openai/codex/pull/13212 to expose fast
tier controls to app server
(majority of this PR is generated schema jsons - actual code is +69 /
-35 and +24 tests )

- add service tier fields to the app-server protocol surfaces used by
thread lifecycle, turn start, config, and session configured events
- thread service tier through the app-server message processor and core
thread config snapshots
- allow runtime config overrides to carry service tier for app-server
callers

cleanup:
- Removing useless "legacy" code supporting "standard" - we moved to
None | "fast", so "standard" is not needed.

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

@@ -299,6 +299,12 @@
         }
       ]
     },
+    "ServiceTier": {
+      "enum": [
+        "fast"
+      ],
+      "type": "string"
+    },
     "Settings": {
       "description": "Settings for a collaboration mode.",
       "properties": {
@@ -539,6 +545,24 @@
       ],
       "description": "Override the sandbox policy for this turn and subsequent turns."
     },
+    "serviceTier": {
+      "anyOf": [
+        {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/ServiceTier"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        {
+          "type": "null"
+        }
+      ],
+      "description": "Override the service tier for this turn and subsequent turns."
+    },
     "summary": {
       "anyOf": [
         {