refactor: centralize TEST_MODELS and DEFAULT_PRESETS in constants.js

- Move TEST_MODELS and DEFAULT_PRESETS to src/constants.js as single source of truth
- Update test-models.cjs helper to use dynamic import from constants
- Make getTestModels() and getModels() async functions
- Update all test files to await async model config loading
- Remove duplicate THINKING_MODELS and getThinkingModels() from test helper
- Make thinking tests more lenient for Gemini (doesn't always produce thinking blocks)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test-empty-response-retry.cjs

@@ -6,9 +6,11 @@
  */
 
 const { streamRequest } = require('./helpers/http-client.cjs');
-const { TEST_MODELS } = require('./helpers/test-models.cjs');
+const { getModels } = require('./helpers/test-models.cjs');
 
 async function testEmptyResponseRetry() {
+    const TEST_MODELS = await getModels();
+
     console.log('\n============================================================');
     console.log('EMPTY RESPONSE RETRY TEST');
     console.log('Tests retry mechanism for empty API responses');