2- Use string service tiers in session protocol (#20971)

## Summary
- break service tier session/op/app-server protocol fields from the
closed enum to string tier ids
- send the service tier string directly through model requests, prewarm,
compaction, memories, and TUI/app-server turn starts
- regenerate app-server protocol JSON/TypeScript schemas, removing the
standalone ServiceTier TS enum

## Verification
- just fmt
- cargo check -p codex-core -p codex-app-server -p codex-tui
- just write-app-server-schema

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -312,13 +312,6 @@
         }
       ]
     },
-    "ServiceTier": {
-      "enum": [
-        "fast",
-        "flex"
-      ],
-      "type": "string"
-    },
     "Settings": {
       "description": "Settings for a collaboration mode.",
       "properties": {
@@ -586,22 +579,11 @@
       "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."
+      "description": "Override the service tier for this turn and subsequent turns.",
+      "type": [
+        "string",
+        "null"
+      ]
     },
     "summary": {
       "anyOf": [