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-multiturn-thinking-tools-streaming.cjs

@@ -165,7 +165,7 @@ drwxr-xr-x   4 user  staff   128 Dec 19 10:00 tests`
 }
 
 async function runTests() {
-    const models = getTestModels();
+    const models = await getTestModels();
     let allPassed = true;
 
     for (const { family, model } of models) {