fix(core): handle non-string model flags in resolution (#26069)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/cli/src/config/config.ts

@@ -841,9 +841,16 @@ export async function loadCliConfig(
   );
 
   const defaultModel = PREVIEW_GEMINI_MODEL_AUTO;
-  const specifiedModel =
+  const rawModel =
     argv.model || process.env['GEMINI_MODEL'] || settings.model?.name;
 
+  // Ensure specifiedModel is a string (e.g. if yargs parsed multiple --model as an array)
+  const specifiedModel = Array.isArray(rawModel)
+    ? String(rawModel.at(-1) ?? '').trim() || ''
+    : rawModel === undefined
+      ? undefined
+      : String(rawModel ?? '').trim() || '';
+
   const resolvedModel =
     specifiedModel === GEMINI_MODEL_ALIAS_AUTO
       ? defaultModel