refactor: Leverage Python project root detection for jest-runner resolution

Instead of duplicating monorepo detection logic in JavaScript, leverage
the existing Python _find_node_project_root and add _find_monorepo_root
functions. Pass the detected monorepo root via CODEFLASH_MONOREPO_ROOT
environment variable to the loop-runner.

This approach:
- Reuses existing Python project detection logic
- Provides a reliable monorepo root hint to JavaScript
- Maintains fallback directory traversal for edge cases

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

codeflash/languages/javascript/test_runner.py

@@ -296,6 +296,33 @@ def _find_node_project_root(file_path: Path) -> Path | None:
     return None
 
 
+def _find_monorepo_root(start_path: Path) -> Path | None:
+    """Find the monorepo workspace root by looking for workspace markers.
+
+    Traverses up from the given path to find a directory containing
+    monorepo workspace markers like yarn.lock, pnpm-workspace.yaml, etc.
+
+    Args:
+        start_path: A path within the monorepo.
+
+    Returns:
+        The monorepo root directory, or None if not found.
+
+    """
+    monorepo_markers = ["yarn.lock", "pnpm-workspace.yaml", "lerna.json", "package-lock.json"]
+    current = start_path if start_path.is_dir() else start_path.parent
+
+    while current != current.parent:
+        # Check for monorepo markers
+        if any((current / marker).exists() for marker in monorepo_markers):
+            # Verify it has node_modules (it's the workspace root)
+            if (current / "node_modules").exists():
+                return current
+        current = current.parent
+
+    return None
+
+
 def _find_jest_config(project_root: Path) -> Path | None:
     """Find Jest configuration file in the project.
 
@@ -797,6 +824,12 @@ def run_jest_benchmarking_tests(
     jest_env["JEST_JUNIT_SUITE_NAME"] = "{filepath}"
     jest_env["JEST_JUNIT_ADD_FILE_ATTRIBUTE"] = "true"
     jest_env["JEST_JUNIT_INCLUDE_CONSOLE_OUTPUT"] = "true"
+
+    # Pass monorepo root to loop-runner for jest-runner resolution
+    monorepo_root = _find_monorepo_root(effective_cwd)
+    if monorepo_root:
+        jest_env["CODEFLASH_MONOREPO_ROOT"] = str(monorepo_root)
+        logger.debug(f"Detected monorepo root: {monorepo_root}")
     codeflash_sqlite_file = get_run_tmp_file(Path("test_return_values_0.sqlite"))
     jest_env["CODEFLASH_OUTPUT_FILE"] = str(codeflash_sqlite_file)
     jest_env["CODEFLASH_TEST_ITERATION"] = "0"

packages/codeflash/runtime/loop-runner.js

@@ -34,7 +34,8 @@ const fs = require('fs');
 
 /**
  * Resolve jest-runner with monorepo support.
- * Walks up the directory tree looking for node_modules/jest-runner.
+ * Uses CODEFLASH_MONOREPO_ROOT environment variable if available,
+ * otherwise walks up the directory tree looking for node_modules/jest-runner.
  */
 function resolveJestRunner() {
     // Try standard resolution first (works in simple projects)
@@ -44,7 +45,19 @@ function resolveJestRunner() {
         // Standard resolution failed - try monorepo-aware resolution
     }
 
-    // Walk up from cwd looking for workspace root markers and node_modules
+    // If Python detected a monorepo root, check there first
+    const monorepoRoot = process.env.CODEFLASH_MONOREPO_ROOT;
+    if (monorepoRoot) {
+        const jestRunnerPath = path.join(monorepoRoot, 'node_modules', 'jest-runner');
+        if (fs.existsSync(jestRunnerPath)) {
+            const packageJsonPath = path.join(jestRunnerPath, 'package.json');
+            if (fs.existsSync(packageJsonPath)) {
+                return jestRunnerPath;
+            }
+        }
+    }
+
+    // Fallback: Walk up from cwd looking for node_modules/jest-runner
     const monorepoMarkers = ['yarn.lock', 'pnpm-workspace.yaml', 'lerna.json', 'package-lock.json'];
     let currentDir = process.cwd();
     const visitedDirs = new Set();