Fix path relative-to for case-insensitive filesystems (#16310)

fixes #16205 

# Description

1. **Adds fallback**: On case-insensitive filesystems (Windows, macOS),
falls back to case-insensitive comparison when the standard comparison
fails
2. **Maintains filesystem semantics**: Only uses case-insensitive
comparison on platforms where it's appropriate

## Before:
```console
$> "/etc" | path relative-to "/Etc"
Error: nu::shell::cant_convert

  × Can't convert to prefix not found.
   ╭─[entry #33:1:1]
 1 │ "/etc" | path relative-to "/Etc"
   · ───┬──
   ·    ╰── can't convert string to prefix not found
   ╰────
```

## After:
For Windows and macOS:
```console
$> "/etc" | path relative-to "/Etc" | debug -v
""
```

Author: hustcer

crates/nu-command/src/path/relative_to.rs

@@ -144,17 +144,84 @@ path."#
 fn relative_to(path: &Path, span: Span, args: &Arguments) -> Value {
     let lhs = expand_to_real_path(path);
     let rhs = expand_to_real_path(&args.path.item);
+
     match lhs.strip_prefix(&rhs) {
         Ok(p) => Value::string(p.to_string_lossy(), span),
-        Err(e) => Value::error(
-            ShellError::CantConvert {
-                to_type: e.to_string(),
-                from_type: "string".into(),
+        Err(e) => {
+            // On case-insensitive filesystems, try case-insensitive comparison
+            if is_case_insensitive_filesystem() {
+                if let Some(relative_path) = try_case_insensitive_strip_prefix(&lhs, &rhs) {
+                    return Value::string(relative_path.to_string_lossy(), span);
+                }
+            }
+
+            Value::error(
+                ShellError::CantConvert {
+                    to_type: e.to_string(),
+                    from_type: "string".into(),
+                    span,
+                    help: None,
+                },
                 span,
-                help: None,
-            },
-            span,
-        ),
+            )
+        }
+    }
+}
+
+/// Check if the current filesystem is typically case-insensitive
+fn is_case_insensitive_filesystem() -> bool {
+    // Windows and macOS typically have case-insensitive filesystems
+    cfg!(any(target_os = "windows", target_os = "macos"))
+}
+
+/// Try to strip prefix in a case-insensitive manner
+fn try_case_insensitive_strip_prefix(lhs: &Path, rhs: &Path) -> Option<std::path::PathBuf> {
+    let mut lhs_components = lhs.components();
+    let mut rhs_components = rhs.components();
+
+    // Compare components case-insensitively
+    loop {
+        match (lhs_components.next(), rhs_components.next()) {
+            (Some(lhs_comp), Some(rhs_comp)) => {
+                match (lhs_comp, rhs_comp) {
+                    (
+                        std::path::Component::Normal(lhs_name),
+                        std::path::Component::Normal(rhs_name),
+                    ) => {
+                        if lhs_name.to_string_lossy().to_lowercase()
+                            != rhs_name.to_string_lossy().to_lowercase()
+                        {
+                            return None;
+                        }
+                    }
+                    // Non-Normal components must match exactly
+                    _ if lhs_comp != rhs_comp => {
+                        return None;
+                    }
+                    _ => {}
+                }
+            }
+            (Some(lhs_comp), None) => {
+                // rhs is fully consumed, but lhs has more components
+                // This means rhs is a prefix of lhs, collect remaining lhs components
+                let mut result = std::path::PathBuf::new();
+                // Add the current lhs component that wasn't matched
+                result.push(lhs_comp);
+                // Add all remaining lhs components
+                for component in lhs_components {
+                    result.push(component);
+                }
+                return Some(result);
+            }
+            (None, Some(_)) => {
+                // lhs is shorter than rhs, so rhs cannot be a prefix of lhs
+                return None;
+            }
+            (None, None) => {
+                // Both paths have the same components, relative path is empty
+                return Some(std::path::PathBuf::new());
+            }
+        }
     }
 }
 
@@ -168,4 +235,89 @@ mod tests {
 
         test_examples(PathRelativeTo {})
     }
+
+    #[test]
+    fn test_case_insensitive_filesystem() {
+        use nu_protocol::{Span, Value};
+        use std::path::Path;
+
+        let args = Arguments {
+            path: Spanned {
+                item: "/Etc".to_string(),
+                span: Span::test_data(),
+            },
+        };
+
+        let result = relative_to(Path::new("/etc"), Span::test_data(), &args);
+
+        // On case-insensitive filesystems (Windows, macOS), this should work
+        // On case-sensitive filesystems (Linux, FreeBSD), this should fail
+        if is_case_insensitive_filesystem() {
+            match result {
+                Value::String { val, .. } => {
+                    assert_eq!(val, "");
+                }
+                _ => panic!("Expected string result on case-insensitive filesystem"),
+            }
+        } else {
+            match result {
+                Value::Error { .. } => {
+                    // Expected on case-sensitive filesystems
+                }
+                _ => panic!("Expected error on case-sensitive filesystem"),
+            }
+        }
+    }
+
+    #[test]
+    fn test_case_insensitive_with_subpath() {
+        use nu_protocol::{Span, Value};
+        use std::path::Path;
+
+        let args = Arguments {
+            path: Spanned {
+                item: "/Home/User".to_string(),
+                span: Span::test_data(),
+            },
+        };
+
+        let result = relative_to(Path::new("/home/user/documents"), Span::test_data(), &args);
+
+        if is_case_insensitive_filesystem() {
+            match result {
+                Value::String { val, .. } => {
+                    assert_eq!(val, "documents");
+                }
+                _ => panic!("Expected string result on case-insensitive filesystem"),
+            }
+        } else {
+            match result {
+                Value::Error { .. } => {
+                    // Expected on case-sensitive filesystems
+                }
+                _ => panic!("Expected error on case-sensitive filesystem"),
+            }
+        }
+    }
+
+    #[test]
+    fn test_truly_different_paths() {
+        use nu_protocol::{Span, Value};
+        use std::path::Path;
+
+        let args = Arguments {
+            path: Spanned {
+                item: "/Different/Path".to_string(),
+                span: Span::test_data(),
+            },
+        };
+
+        let result = relative_to(Path::new("/home/user"), Span::test_data(), &args);
+
+        // This should fail on all filesystems since paths are truly different
+        match result {
+            Value::Error { .. } => {}
+            _ => panic!("Expected error for truly different paths"),
+        }
+    }
 }