Use model metadata for Fast Mode status (#16949)

Fast Mode status was still tied to one model name in the TUI and
model-list plumbing. This changes the model metadata shape so a model
can advertise additional speed tiers, carries that field through the
app-server model list, and uses it to decide when to show Fast Mode
status.

For people using Codex, the behavior is intended to stay the same for
existing models. Fast Mode still requires the existing signed-in /
feature-gated path; the difference is that the UI can now recognize any
model the model list marks as Fast-capable, instead of requiring a new
client-side slug check.

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -6111,6 +6111,13 @@
     },
     "Model": {
       "properties": {
+        "additionalSpeedTiers": {
+          "default": [],
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
+        },
         "availabilityNux": {
           "anyOf": [
             {