Return a list when listing a single GitHub file

[bIackr0se]

Listing a single file with GithubFileSystem.ls() returns a string instead of a list. Code that iterates the result sees individual characters rather than a filename. Return a one-element list using the resolved file entry, consistent with detailed listings.

fs.ls("README.md")
# before: "README.md"
# after:  ["README.md"]

Verification:

• Four non-detailed listing regressions fail before the fix and pass after it; detailed listings, directory listings and missing-file errors remain covered.
• GitHub listing/path tests and the filesystem specification suite: 241 passed.
• Installed wheel against the public GitHub API at 778f956: root and nested file listings, protocol-prefixed inputs, listing-to-read iteration, directory listings and missing-file handling pass.
• Pre-commit checks pass.