feat(providers): add Vercel AI Gateway (vercel-ai-gateway)

12th provider. API-key via AI_GATEWAY_API_KEY, Bearer, OpenAI listing +
ChatCompletions, Passthrough dialect. Second wire-metadata provider but
takes the enrichment path instead: Vercel serves context under
context_window, which WireModel ignores (reads context_length), so
wire_serves_metadata=false + restrict_to_enriched pulls context/limits
from the models.dev "vercel" snapshot (302/306 live ids match snapshot
keys byte-for-byte, so restrict keeps essentially the whole catalog).

/models is public (200 for any key), so login validation targets
/credits (key_validation_path) which 401s on a bad bearer — avoids
false-accepting invalid keys against the public listing.

Tests: e2e proves enrichment-wins (wire context_window=999 distinct from
enrichment context=400000, asserts 400000) and non-vacuous tool_call
restriction; validation test proves /credits (not /models) is hit.
Registry at 15 (ordinal/VARIANT_COUNT/ALL), 4 auth arrays + 16-row picker.

crates/codegen/kigi-tui/src/app/app_view.rs

@@ -6894,7 +6894,7 @@ pub(crate) mod tests {
     #[test]
     fn pending_menu_items_lists_interactive_methods_plus_quit() {
         let items = pending_menu_items(&fresh_user_auth_methods(), None);
-        assert_eq!(items.len(), 15, "14 login rows + Quit, got {items:?}");
+        assert_eq!(items.len(), 16, "15 login rows + Quit, got {items:?}");
         assert!(
             matches!(&items[0], PendingMenuItem::Login { label } if label == "Kimi Code (OAuth)"),
             "row 0 must be the OAuth login, got {:?}",
@@ -6992,7 +6992,14 @@ pub(crate) mod tests {
                 label: "NVIDIA NIM (API key)".into(),
             }
         );
-        assert_eq!(items[14], PendingMenuItem::Quit);
+        assert_eq!(
+            items[14],
+            PendingMenuItem::ApiKey {
+                target: PlatformLogin(kigi_shell::models::PlatformId::Vercel),
+                label: "Vercel AI Gateway (API key)".into(),
+            }
+        );
+        assert_eq!(items[15], PendingMenuItem::Quit);
         // The non-interactive methods must never appear as rows.
         let byok = kigi_shell::agent::auth_method::build_auth_methods(
             kigi_shell::agent::auth_method::AuthMethodsBuildInputs {
@@ -7003,7 +7010,7 @@ pub(crate) mod tests {
         );
         assert_eq!(
             pending_menu_items(&byok.methods, None).len(),
-            15,
+            16,
             "xai.api_key / cached_token must not add rows"
         );
     }