Fix markdown image hover preview for paths exceeding MAX_PATH on Windows

[yogeshwaran-c]

What / Why

On Windows, the markdown image hover preview fails when a file's absolute path exceeds MAX_PATH (259 characters), even though the full preview panel renders the image correctly. The root cause is that the hover preview loads images through Electron's registerFileProtocol (the vscode-file:// handler in ProtocolMainService), and Electron's handler bottoms out in a Win32 API call that does not use the extended-length path prefix — so anything over MAX_PATH is rejected by the OS. This was diagnosed in the issue thread by @busorgin and tracked upstream at electron/electron#49101.

Fixes #261880

Fix

Before handing a resolved path to Electron's protocol callback, transform it on Windows to the extended-length form:

• C:\... becomes \?\C:\...
• \server\share\... becomes \?\UNC\server\share\...

This opts the underlying Win32 APIs into extended-length path mode, which bypasses the MAX_PATH limit. The transformation is:

• Windows-only — no-op on macOS and Linux, where the limit does not apply.
• Only applied past a conservative threshold (248 chars) so normal short paths are passed through unchanged and untouched behavior is preserved.
• Idempotent — already-prefixed paths are returned as-is.
• Applied only to the value handed to Electron. The validRoots / validExtensions allow-list checks continue to run against the original normalized path, so the security boundary around vscode-file:// is unchanged.

Testing

• Manually verified on Windows 11 by creating a markdown file that references an image whose absolute path is ~290 characters. Before the fix, hovering the image link showed the broken-image placeholder. After the fix, the hover preview renders the image, matching the behavior of the full preview panel.
• Short paths continue to work unchanged; the transformation is skipped for any path under the threshold.
• UNC paths, drive-letter paths, and already-prefixed paths are each handled explicitly and covered by the guard branches in toWindowsLongPathIfNeeded.

Notes

• This is a workaround for Unable to load images on Windows from paths over 260 characters electron/electron#49101. If/when Electron starts applying the extended-length prefix internally, this helper can be removed.