fix(core): resolve 500 errors with gemini-3-flash-preview model

- Refactor default model configs to prevent Flash models from inheriting thinkingConfig
- Add synthetic model turn to initial chat history to enforce alternating user/model roles required by gemini-3-flash-preview
- Fix trailing newline assertion error in file-system integration test

packages/test-utils/src/test-rig.ts

@@ -11,7 +11,10 @@ import { join, dirname } from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { env } from 'node:process';
 import { setTimeout as sleep } from 'node:timers/promises';
-import { DEFAULT_GEMINI_MODEL, GEMINI_DIR } from '@google/gemini-cli-core';
+import {
+  GEMINI_DIR,
+  PREVIEW_GEMINI_FLASH_MODEL,
+} from '@google/gemini-cli-core';
 export { GEMINI_DIR };
 import * as pty from '@lydell/node-pty';
 import stripAnsi from 'strip-ansi';
@@ -457,7 +460,7 @@ export class TestRig {
         ...(env['GEMINI_TEST_TYPE'] === 'integration'
           ? {
               model: {
-                name: DEFAULT_GEMINI_MODEL,
+                name: PREVIEW_GEMINI_FLASH_MODEL,
               },
             }
           : {}),