fix: preserve Windows Explorer new tab directory

[lllakshit]

Description

Fixes #9844.

Windows Explorer's Open Warp in new tab registry entries now pass the selected folder/background folder as a file:///... URI, which routes through Warp's existing file URI handling and preserves the target directory when creating the session.

This also hardens the custom action/new_tab?path=... parser so it can handle shell-quoted paths and file:// path values, and updates Windows single-instance forwarding to append the current directory through Url::query_pairs_mut() instead of concatenating an unescaped query string.

Linked Issue

• The linked issue is labeled ready-to-implement.
• Fixes Bug: 'Open Warp in New Tab' from Explorer context menu doesn't navigate to current directory on Windows #9844

Testing

• git diff --check
• Added unit coverage for quoted tab paths and file:// tab path values.

Not run locally because this Windows machine does not currently have the required developer tools available on PATH:

• cargo
• rustfmt
• Inno Setup iscc

Agent Mode

• Warp Agent Mode - This PR was created via Codex CLI.

CHANGELOG-BUG-FIX: Fixed Windows Explorer's Open Warp in new tab context menu action losing the selected folder and opening at the home directory.

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