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/ThreadStartResponse.json

@@ -738,6 +738,12 @@
         }
       ]
     },
+    "ServiceTier": {
+      "enum": [
+        "fast"
+      ],
+      "type": "string"
+    },
     "SessionSource": {
       "oneOf": [
         {
@@ -1906,6 +1912,16 @@
     "sandbox": {
       "$ref": "#/definitions/SandboxPolicy"
     },
+    "serviceTier": {
+      "anyOf": [
+        {
+          "$ref": "#/definitions/ServiceTier"
+        },
+        {
+          "type": "null"
+        }
+      ]
+    },
     "thread": {
       "$ref": "#/definitions/Thread"
     }