Guard against UNC path bypass in file URI parsing on Windows

[RacerZ-fighting]

Summary

file_uri_to_path() determines whether a file: URI refers to a local resource by
inspecting the parsed URI authority (netloc). However, a crafted URI such as
file:////RemoteServer/Share/file encodes a UNC path entirely within the path component,
causing urlparse() to return an empty netloc — bypassing the locality check — while
url2pathname() on Windows converts it into a valid UNC path (\\RemoteServer\Share\file).

This allows the application to initiate unintended outbound SMB connections when processing
such inputs, which is particularly concerning in automated or agent-driven workflows
(e.g., MCP-based systems) where an attacker could influence input data by hosting
attacker-controlled files on a remote server.

Changes

• Add UNC path detection in the URI path component on Windows (os.name == 'nt'),
  rejecting URIs where the path resolves to a remote UNC share despite an empty netloc
• Simplify server name extraction logic (unify separators before split)
• Remove redundant emptiness check

[RacerZ-fighting]

@microsoft-github-policy-service agree