fix: regenerate stale vitest config and add .js extension for ESM imports

Two fixes discovered during end-to-end testing:

1. _ensure_codeflash_vitest_config() reused existing configs that lacked
   pool: 'forks', causing benchmarking to fall back to wall-clock timing
   on re-runs. Now checks for the required setting and regenerates if
   missing.

2. ESM projects require explicit .js file extensions in import specifiers
   (e.g., `import foo from './lib/foo.js'`). The verifier stripped the
   extension but never added .js back for ESM, causing
   ERR_MODULE_NOT_FOUND in Mocha ESM projects like axios.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Saga4

codeflash/languages/javascript/vitest_runner.py

@@ -194,10 +194,13 @@ def _ensure_codeflash_vitest_config(project_root: Path) -> Path | None:
 
     codeflash_config_path = project_root / "codeflash.vitest.config.mjs"
 
-    # If already exists, use it
+    # Regenerate if existing config is missing required settings (e.g., pool: 'forks')
     if codeflash_config_path.exists():
-        logger.debug(f"Using existing Codeflash Vitest config: {codeflash_config_path}")
-        return codeflash_config_path
+        existing_content = codeflash_config_path.read_text(encoding="utf-8")
+        if "pool: 'forks'" in existing_content:
+            logger.debug(f"Using existing Codeflash Vitest config: {codeflash_config_path}")
+            return codeflash_config_path
+        logger.debug("Regenerating Codeflash Vitest config (missing pool: 'forks')")
 
     # Find the original vitest config to extend
     original_config = None

codeflash/verification/verifier.py

@@ -52,6 +52,10 @@ def generate_tests(
         # Ensure path starts with ./ or ../ for JavaScript/TypeScript imports
         if not rel_import_path.startswith("../"):
             rel_import_path = f"./{rel_import_path}"
+        # ESM requires explicit file extensions in import specifiers.
+        # TypeScript ESM also uses .js extensions (TS resolves .js → .ts).
+        if project_module_system == "esm":
+            rel_import_path += ".js"
         # Keep as string since Path() normalizes away the ./ prefix
         module_path = rel_import_path