Allow string service tiers in config TOML (#21697)

## Why

`service_tier` in `config.toml` and profile config was still modeled as
an enum, which blocked newer or experimental service tier IDs even
though the runtime paths already carry string IDs.

This change makes the TOML-facing config accept string service tier IDs
directly while keeping the legacy `fast` alias behavior by normalizing
it to the request value `priority`.

## What Changed

- change the TOML-facing `service_tier` fields in global and profile
config to `Option<String>`
- keep config-load normalization so legacy `fast` still resolves to
`priority`
- persist resolved service tier strings directly in config locks so
arbitrary IDs round-trip cleanly
- regenerate the config schema and add config coverage for arbitrary
string IDs plus legacy `fast` normalization

## Verification

- added config tests for arbitrary string service tiers and legacy
`fast` normalization
- ran `just write-config-schema`
- CI

---------

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

codex-rs/core/config.schema.json

@@ -670,12 +670,8 @@
           "$ref": "#/definitions/SandboxMode"
         },
         "service_tier": {
-          "allOf": [
-            {
-              "$ref": "#/definitions/ServiceTier"
-            }
-          ],
-          "description": "Optional explicit service tier preference for new turns (`fast` or `flex`)."
+          "description": "Optional explicit service tier request id for new turns (for example `priority` or `flex`; legacy `fast` also works).",
+          "type": "string"
         },
         "tools": {
           "$ref": "#/definitions/ToolsToml"
@@ -2221,13 +2217,6 @@
       },
       "type": "object"
     },
-    "ServiceTier": {
-      "enum": [
-        "fast",
-        "flex"
-      ],
-      "type": "string"
-    },
     "SessionPickerViewMode": {
       "description": "Preferred layout for the resume/fork session picker.",
       "enum": [
@@ -4502,12 +4491,8 @@
       "description": "Sandbox configuration to apply if `sandbox` is `WorkspaceWrite`."
     },
     "service_tier": {
-      "allOf": [
-        {
-          "$ref": "#/definitions/ServiceTier"
-        }
-      ],
-      "description": "Optional explicit service tier preference for new turns (`fast` or `flex`)."
+      "description": "Optional explicit service tier request id for new turns (for example `priority` or `flex`; legacy `fast` also works).",
+      "type": "string"
     },
     "shell_environment_policy": {
       "allOf": [