fix(ui): inherit default model fallbacks in agents overview (#25729)

Land PR #25729 from @Suko.
Use shared fallback-resolution helper and add regression coverage for default, override, and explicit-empty cases.

Co-authored-by: suko <miha.sukic@gmail.com>

vitest.config.ts

@@ -37,6 +37,7 @@ export default defineConfig({
       "src/**/*.test.ts",
       "extensions/**/*.test.ts",
       "test/**/*.test.ts",
+      "ui/src/ui/views/agents-utils.test.ts",
       "ui/src/ui/views/usage-render-details.test.ts",
       "ui/src/ui/controllers/agents.test.ts",
     ],