fix(gitignore): escape directory names before interpolating into pattern position

[AmirF194]

Root cause

GitignoreParser._parse_gitignore_content rescopes patterns from a nested .gitignore by
joining the gitignore directory's relative path (rel_dir) onto each pattern line (three
join sites, src/serena/util/file_system.py:263,269,272 at HEAD 29d07d4f). rel_dir comes
from os.path.relpath and is a filesystem name, but it lands directly in pattern position,
where pathspec's GitWildMatchPattern reads *, ?, [, ], ! and # as syntax rather
than literal characters.

A directory whose name contains one of those characters (the concrete case in the issue: a
stray venv literally named ***, whose own .gitignore is just *) turns a scoped pattern
into a much broader one: ***/**/* reads as "any first segment, then anything at depth >= 2",
so it ends up excluding nearly the whole project. Indexing then reports 0 files with exit code
0, and nothing points at the actual cause.

Invariant

A pattern segment built from a filesystem name must match that name literally, never as glob
syntax. The fix escapes each path component of rel_dir (backslash before \ * ? [ ] ! #,
following pathspec's own escaping convention) at all three join sites before it is folded
into the pattern string, so directory names are always matched literally regardless of their
content.

Verification

• Added three regression tests in test/serena/util/test_file_system.py, one per join site
  (anchored, **/-prefixed non-anchored, and the implicit-** non-anchored case), plus the
  issue's own *** scenario. Ran in a clean python:3.11-slim container against HEAD
  29d07d4f: all three fail on unmodified main and pass on this branch.
• uv run poe test test/serena/util/test_file_system.py: 27/27 pass (24 existing + 3 new).
• uv run poe lint and uv run poe type-check: clean. codespell on the changed files: clean.
• Not run: the full CI matrix (language-server batches across ubuntu/windows/macos) and
  test/serena/util/test_exception.py's three pre-existing GUI-dependent failures, which are
  unrelated to this change and reproduce identically on unmodified main in this container.

Fixes #1806

Checklist

• This PR follows the guidelines in CONTRIBUTING.md regarding the scope of PRs.
• For changes that add features or fix problems, I have added an entry to CHANGELOG.md, which concisely describes the change.

Update (second commit)

The initial Windows CI run (before any review) failed 3 new tests, not the 3 unrelated
pre-existing flakes documented above: test_gitignore_dir_name_with_metachars_anchored_pattern
failed with a real AssertionError (not the platform-limitation OSError the other two
metachar tests hit for */?, which are illegal in Windows filenames). Root cause: the
escaping added above inserts literal backslash escape characters, and on Windows os.sep is
also backslash, so os.path.join(rel_dir_pattern, line) mixed path-separator backslashes with
escape backslashes; the pre-existing trailing .replace(os.sep, "/") then converted all of
them indiscriminately, turning an escaped a\\[1\\] into a/[1/], which no longer matches
the literal directory name.

Fixed by building every pattern with a literal / from the start (never os.sep/
os.path.join), and removing the now-unneeded (and on Windows actively harmful) blanket
separator replace. Verified with ntpath (Windows path semantics) on this Linux session:
simulating the old chain on a Windows-style relpath produces a/[1/]/mod.py (does not match
a[1]/mod.py via pathspec); the new construction produces a\\[1\\]/mod.py (matches).
The two tests that create a directory literally named ***/q? are marked
skipif(sys.platform == "win32") (those characters can't exist in a Windows filename, a test
limitation, not a fix limitation); added a pure-function test for
_escape_gitignore_path_component so that escaping logic keeps cross-platform coverage without
touching the filesystem.