fix(core): improve chat history merging and switch to stable preview flash model

- Refactor GeminiChat history merging to handle consecutive turns of any role.
- Implement part-level filtering to remove empty text segments while preserving tool calls.
- Switch integration tests to 'gemini-3.1-flash-lite-preview' for improved stability.
- Ensure crash report context follows alternating role rules.

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

@@ -12,7 +12,7 @@ import { fileURLToPath } from 'node:url';
 import { env } from 'node:process';
 import { setTimeout as sleep } from 'node:timers/promises';
 import {
-  DEFAULT_GEMINI_FLASH_MODEL,
+  PREVIEW_GEMINI_3_1_FLASH_LITE_MODEL,
   GEMINI_DIR,
 } from '@google/gemini-cli-core';
 export { GEMINI_DIR };
@@ -460,7 +460,7 @@ export class TestRig {
         ...(env['GEMINI_TEST_TYPE'] === 'integration'
           ? {
               model: {
-                name: DEFAULT_GEMINI_FLASH_MODEL,
+                name: PREVIEW_GEMINI_3_1_FLASH_LITE_MODEL,
               },
             }
           : {}),