fix: always warn when pattern contains path separator, not only for existing dirs

[kadaliao]

Problem

Fixes #1873.

ensure_search_pattern_is_not_a_path only showed the path-separator warning when the pattern happened to be an existing directory (Path::new(&opts.pattern).is_dir()). Patterns containing / that do not correspond to an existing directory silently produced no results.

Reproducer (before fix):

mkdir -p dir1/dir2
fd dir1/dir2   # ✅ warns (existing dir)
fd r1/dir2     # ❌ silent, no results (non-existing path)
fd some/nonexistent/pattern  # ❌ silent, no results

Fix

Remove the is_dir() guard so the warning fires whenever the pattern contains a path separator and --full-path is not set.

Windows caveat: \ is simultaneously a path separator and a valid regex escape character (\d, \w, etc.) on Windows. To avoid false-positive warnings on legitimate regex patterns, the fix only checks for / on Windows.

let pattern_has_separator = if cfg!(windows) {
    opts.pattern.contains('/')
} else {
    opts.pattern.contains(std::path::MAIN_SEPARATOR)
};

Tests

Added 4 unit tests for ensure_search_pattern_is_not_a_path:

• Pattern with separator, non-existent path → now errors ✅
• Pattern with separator, existing dir → still errors ✅
• Pattern without separator → Ok(()) ✅
• --full-path flag suppresses warning → Ok(()) ✅

All 242 tests pass (cargo test).