[Windows] Fix Copy Path and Copy Relative Path returning identical results. (marimo-team#5399)

## 📝 Summary

Fix marimo-team#5395 

## 🔍 Description of Changes

Both copy path options in the file explorer's (View File sidebar panel)
context menu were returning the same absolute path on Windows systems.
The relative path should show only the portion relative to the workspace
root, not the full/absolute path.

Fixed by replacing the slash logic with a new `withTrailingDelimiter`
method that uses the existing `PathBuilder.deliminator` to append the
correct path separator accordingly.

Added tests covering both Unix and Windows path scenarios.

**Before**: Both options returned `D:\full\path\to\file.py`  
**After**: Copy Path returns `D:\full\path\to\file.py`, Copy Relative
Path returns `subfolder\file.py`

## Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
[Discord](https://marimo.io/discord?ref=pr), or the community
[discussions](https://github.com/marimo-team/marimo/discussions) (Please
provide a link if applicable).
- [x] I have added tests for the changes made.
- [x] I have run the code and verified that it works as expected.

## 📜 Reviewers

@akshayka OR @Light2Dark

---------

Signed-off-by: Srihari Thyagarajan <hari.leo03@gmail.com>
Co-authored-by: Shahmir Varqha <sham9871@gmail.com>

Author: 2 people

frontend/src/components/editor/file-tree/__tests__/requesting-tree.test.ts

@@ -1,6 +1,7 @@
 /* Copyright 2024 Marimo. All rights reserved. */
 import { afterEach, beforeEach, describe, expect, test, vi } from "vitest";
 import { toast } from "@/components/ui/use-toast";
+import type { FilePath } from "@/utils/paths";
 import { RequestingTree } from "../requesting-tree";
 
 const sendListFiles = vi.fn();
@@ -261,4 +262,60 @@ describe("RequestingTree", () => {
       expect(mockOnChange).toHaveBeenCalled();
     });
   });
+
+  describe("relativeFromRoot", () => {
+    test("should return relative path for Unix paths", async () => {
+      const tree = new RequestingTree({
+        listFiles: sendListFiles,
+        createFileOrFolder: sendCreateFileOrFolder,
+        deleteFileOrFolder: sendDeleteFileOrFolder,
+        renameFileOrFolder: sendRenameFileOrFolder,
+      });
+
+      sendListFiles.mockResolvedValue({
+        files: [],
+        root: "/home/user/project",
+      });
+
+      await tree.initialize(vi.fn());
+
+      const relativePath = tree.relativeFromRoot(
+        "/home/user/project/src/file.py" as FilePath,
+      );
+      expect(relativePath).toBe("src/file.py");
+    });
+
+    test("should return relative path for Windows paths", async () => {
+      const tree = new RequestingTree({
+        listFiles: sendListFiles,
+        createFileOrFolder: sendCreateFileOrFolder,
+        deleteFileOrFolder: sendDeleteFileOrFolder,
+        renameFileOrFolder: sendRenameFileOrFolder,
+      });
+
+      sendListFiles.mockResolvedValue({
+        files: [],
+        root: "C:\\Users\\test\\project",
+      });
+
+      await tree.initialize(vi.fn());
+
+      const relativePath = tree.relativeFromRoot(
+        "C:\\Users\\test\\project\\src\\file.py" as FilePath,
+      );
+      expect(relativePath).toBe("src\\file.py");
+    });
+
+    test("should return original path when not under root", async () => {
+      const relativePath = requestingTree.relativeFromRoot(
+        "/other/path/file.py" as FilePath,
+      );
+      expect(relativePath).toBe("/other/path/file.py");
+    });
+
+    test("should handle root path exactly", async () => {
+      const relativePath = requestingTree.relativeFromRoot("/root" as FilePath);
+      expect(relativePath).toBe("/root");
+    });
+  });
 });

frontend/src/components/editor/file-tree/requesting-tree.tsx

@@ -203,7 +203,11 @@ export class RequestingTree {
   };
 
   public relativeFromRoot = (path: FilePath): FilePath => {
-    const root = withTrailingSlash(this.rootPath);
+    // Add a trailing delimiter to the root path if it doesn't have one
+    const root = this.rootPath.endsWith(this.path.deliminator)
+      ? this.rootPath
+      : `${this.rootPath}${this.path.deliminator}`;
+
     if (path.startsWith(root)) {
       return path.slice(root.length) as FilePath;
     }
@@ -224,7 +228,3 @@ export class RequestingTree {
     return response;
   };
 }
-
-function withTrailingSlash(path: string): string {
-  return path.endsWith("/") ? path : `${path}/`;
-}