fix(adk): normalize path separators in InMemoryBackend for Windows compatibility

On Windows, filepath.Clean converts forward slashes to backslashes,
but all path operations in LsInfo, GlobInfo, GrepRaw, Read, Write, and
Edit hardcode "/" as the path separator. This mismatch causes every
prefix check to fail, making the InMemoryBackend completely unusable
on Windows.

The fix wraps filepath.Clean with filepath.ToSlash to ensure paths
always use forward slashes internally, matching the hardcoded "/"
separators used throughout the backend.

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

Author: mjlzz

adk/filesystem/backend_inmemory.go

@@ -697,7 +697,8 @@ func normalizePath(path string) string {
 		path = "/" + path
 	}
 
-	return filepath.Clean(path)
+	// Use ToSlash so paths always use forward-slash internally, regardless of OS.
+	return filepath.ToSlash(filepath.Clean(path))
 }
 
 type grepCollector struct {