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/ConfigReadResponse.json

@@ -323,6 +323,16 @@
             }
           ]
         },
+        "service_tier": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/ServiceTier"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "tools": {
           "anyOf": [
             {
@@ -608,6 +618,16 @@
             }
           ]
         },
+        "service_tier": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/ServiceTier"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "web_search": {
           "anyOf": [
             {
@@ -685,6 +705,12 @@
       },
       "type": "object"
     },
+    "ServiceTier": {
+      "enum": [
+        "fast"
+      ],
+      "type": "string"
+    },
     "ToolsV2": {
       "properties": {
         "view_image": {