fix: correct runtime jest config bugs

- Add encoding="utf-8" to write_text (CLAUDE.md requirement)
- Fix return type to Path | None and return None on write failure with no base config
- Replace startswith path prefix check with Path.is_relative_to()

Co-authored-by: mohammed ahmed <undefined@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people

codeflash/languages/javascript/test_runner.py

@@ -339,7 +339,7 @@ def _create_codeflash_jest_config(
         return None
 
 
-def _create_runtime_jest_config(base_config_path: Path | None, project_root: Path, test_dirs: set[str]) -> Path:
+def _create_runtime_jest_config(base_config_path: Path | None, project_root: Path, test_dirs: set[str]) -> Path | None:
     """Create a runtime Jest config that includes test directories in roots and testMatch.
 
     This is needed because test files generated by codeflash may be placed
@@ -393,16 +393,15 @@ def _create_runtime_jest_config(base_config_path: Path | None, project_root: Pat
 """
 
     try:
-        runtime_config_path.write_text(config_content)
+        runtime_config_path.write_text(config_content, encoding="utf-8")
         _created_config_files.add(runtime_config_path)
         logger.debug(f"Created runtime Jest config with test roots: {runtime_config_path}")
     except Exception as e:
         logger.warning(f"Failed to create runtime Jest config: {e}")
         # Fall back to base config
         if base_config_path:
             return base_config_path
-        # Return a dummy path that won't be used (caller checks for None)
-        return runtime_config_path
+        return None
 
     return runtime_config_path
 
@@ -786,9 +785,9 @@ def run_jest_behavioral_tests(
     # This is necessary because Jest's testMatch patterns use <rootDir> which
     # resolves to the config file's directory, excluding external test files.
     if test_files:
+        resolved_root = effective_cwd.resolve()
         test_dirs = {str(Path(f).resolve().parent) for f in test_files}
-        project_root_str = str(effective_cwd.resolve())
-        if any(not d.startswith(project_root_str) for d in test_dirs):
+        if any(not Path(d).is_relative_to(resolved_root) for d in test_dirs):
             jest_config = _create_runtime_jest_config(jest_config, effective_cwd, test_dirs)
 
     if jest_config:
@@ -1038,9 +1037,9 @@ def run_jest_benchmarking_tests(
 
     # If test files are outside the project root, create a runtime wrapper config
     if test_files:
+        resolved_root = effective_cwd.resolve()
         test_dirs = {str(Path(f).resolve().parent) for f in test_files}
-        project_root_str = str(effective_cwd.resolve())
-        if any(not d.startswith(project_root_str) for d in test_dirs):
+        if any(not Path(d).is_relative_to(resolved_root) for d in test_dirs):
             jest_config = _create_runtime_jest_config(jest_config, effective_cwd, test_dirs)
 
     if jest_config:
@@ -1207,9 +1206,9 @@ def run_jest_line_profile_tests(
 
     # If test files are outside the project root, create a runtime wrapper config
     if test_files:
+        resolved_root = effective_cwd.resolve()
         test_dirs = {str(Path(f).resolve().parent) for f in test_files}
-        project_root_str = str(effective_cwd.resolve())
-        if any(not d.startswith(project_root_str) for d in test_dirs):
+        if any(not Path(d).is_relative_to(resolved_root) for d in test_dirs):
             jest_config = _create_runtime_jest_config(jest_config, effective_cwd, test_dirs)
 
     if jest_config: