Return False from isValidFont for a missing font (#156)

[apoorvdarshan]

Fixes #156.

Problem

FigletFont.isValidFont(font) is a validity predicate, but it raises FileNotFoundError for a .flf/.tlf-named font that doesn't exist, instead of returning False:

>>> from pyfiglet import FigletFont
>>> FigletFont.isValidFont("slantd.flf")
Traceback (most recent call last):
  ...
  File ".../pyfiglet/__init__.py", line 182, in isValidFont
    f = importlib.resources.files('pyfiglet.fonts').joinpath(font).open('rb')
FileNotFoundError: [Errno 2] No such file or directory: '.../pyfiglet/fonts/slantd.flf'

When the font is neither a local file, nor in SHARED_DIRECTORY, nor a bundled package resource, the fallback ...joinpath(font).open('rb') raises. Callers reasonably expect isValidFont to simply return False for an unknown font.

Fix

Wrap the font-opening in try/except FileNotFoundError and return False when the font can't be found:

try:
    if os.path.isfile(font):
        f = open(font, 'rb')
    elif os.path.isfile(full_file):
        f = open(full_file, 'rb')
    else:
        f = importlib.resources.files('pyfiglet.fonts').joinpath(font).open('rb')
except FileNotFoundError:
    return False

Testing

• Added test_is_valid_font_missing: isValidFont("does_not_exist.flf") and .tlf return False (no exception), and a real bundled font (standard.flf) is still recognized as valid. It fails on master (raises FileNotFoundError) and passes with this change.
• The test suite passes (4 passed with the pyfiglet entry point on PATH; the CLI subprocess tests need it there, which is a pre-existing environment requirement unrelated to this change).

---

Disclosure: this change was prepared with the assistance of an AI tool (Claude Code). I reproduced the issue, implemented and verified the fix and test, and take responsibility for the contribution and will respond to review feedback personally.

[pwaller]

I just landed #158 which conflicts with the PR you sent. Can you ask your AI to rebase against it and fix up the conflicts?