fix: show glob-specific error for path separators in --glob patterns (fixes #1720)

[wyf027]

Summary

• When --glob is used with a pattern containing /, show a glob-specific error message
• Points users to --glob --full-path for matching against full paths

Motivation

Fixes #1720. The existing path-separator diagnostic is regex-oriented; glob users benefit from guidance that mentions --glob --full-path explicitly.

Example

$ fd --glob 'foo/*.rs'
[fd error]: The glob pattern 'foo/*.rs' contains a path separator.

By default, `fd --glob` matches file names only, not directory paths. To match against the full path, use:

  fd --glob --full-path 'foo/*.rs'

Test plan

• cargo test test_glob_pattern_with_forward_slash_is_rejected