editor: Fix file URL navigation in hover popover on Windows

[MZZW]

Use Url::to_file_path() instead of PathBuf::from(uri.path()) in
open_markdown_url. On Windows, url::Url::path() returns paths with a
leading slash (e.g., /D:/path/to/file.rs), which PathBuf::from()
cannot resolve correctly. This caused LSP hover links (file:// URIs from
language servers) to fail navigation on Windows.

Url::to_file_path() handles the platform-specific path conversion
properly - it strips the leading slash on Windows and returns the
correct D:\path\to\file format.

Objective

Fix LSP hover popover file links failing to navigate when clicking on
Windows. The root cause is that PathBuf::from(uri.path()) does not
handle the leading / in the URL path on Windows.

Solution

Use url::Url::to_file_path() instead of PathBuf::from(uri.path()).
to_file_path() is the standard, cross-platform way to convert a
file:// URL to a native PathBuf — it correctly strips the leading
slash on Windows and produces proper D:\... paths.

Testing

• Tested manually by inspecting the parsed path values on a Windows
  machine.
• The fix is mechanical (one-liner replacement) and only affects the
  Windows code path. Non-Windows platforms remain unaffected.
• Added unit test test_file_url_to_path_conversion_on_windows in
  hover_popover.rs that verifies Url::to_file_path() produces the
  correct path.
• Existing tests in mention.rs (test_parse_file_uris_use_native_separators_on_windows)
  cover the same underlying path conversion logic and pass.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• The content adheres to Zed's UI standards
• Tests cover the new/changed behavior (unit test in hover_popover.rs)
• Performance impact has been considered and is acceptable

---

Release Notes:

• Fixed LSP hover link navigation failing on Windows due to incorrect file path resolution