fix: normalize paths to prevent concatenation on Windows

Fixes #5673

On Windows, when using absolute paths like C:\path\to\dir,
the path could be incorrectly concatenated, resulting in:
C:\working\dir\C:\path\to\dir\file

This fix applies filepath.Abs() and filepath.Clean() to normalize
the path before using it, ensuring absolute paths are handled
correctly on all platforms.

Changes:
- Apply filepath.Abs() to convert relative paths to absolute
- Apply filepath.Clean() to remove redundant separators and resolve ..
- Add tests for absolute paths, complex paths with .., and paths
  with redundant separators to verify normalization works correctly

The tests use actual 'flux build kustomization' commands with:
1. Absolute paths (prevents concatenation bugs)
2. Paths with parent directory (..) references
3. Paths with redundant separators (//)

All tests verify the command produces correct output, ensuring
the path normalization fix works as expected.

Signed-off-by: Sibasis Padhi <sibasis.padhi@gmail.com>

cmd/flux/build_kustomization.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"os"
 	"os/signal"
+	"path/filepath"
 
 	"github.com/spf13/cobra"
 
@@ -97,6 +98,13 @@ func buildKsCmdRun(cmd *cobra.Command, args []string) (err error) {
 		return fmt.Errorf("invalid resource path %q", buildKsArgs.path)
 	}
 
+	// Normalize the path to handle Windows absolute and relative paths correctly
+	buildKsArgs.path, err = filepath.Abs(buildKsArgs.path)
+	if err != nil {
+		return fmt.Errorf("failed to resolve absolute path: %w", err)
+	}
+	buildKsArgs.path = filepath.Clean(buildKsArgs.path)
+
 	if fs, err := os.Stat(buildKsArgs.path); err != nil || !fs.IsDir() {
 		return fmt.Errorf("invalid resource path %q", buildKsArgs.path)
 	}