Problem

On systems where only soffice is on PATH (the typical case on macOS after
brew install --cask libreoffice), the converter loop logged a WARNING
for the libreoffice candidate before immediately succeeding via the
soffice fallback. Users saw:

WARNING:raganything.parser:LibreOffice command 'libreoffice' not found
INFO:raganything.parser:Successfully converted thaiculture.pptx to PDF using soffice

This made the conversion look broken (#230) even though it completed
successfully. Users were confused and opened support questions.

Root Cause

FileNotFoundError in the candidate loop was always logged at WARNING
level, regardless of whether more candidates remained to try.

Fix

Introduce an is_last flag and only emit a WARNING when the
FileNotFoundError is raised for the final candidate (i.e. all options
are exhausted). For intermediate candidates the message is demoted to
DEBUG, keeping normal logs clean while still allowing the full trace to
appear under --debug.

# Before — always WARNING
cls.logger.warning(f"LibreOffice command '{cmd}' not found")

# After — DEBUG for non-final, WARNING only when all candidates fail
if is_last:
    cls.logger.warning(f"LibreOffice command '{cmd}' not found")
else:
    cls.logger.debug(f"LibreOffice command '{cmd}' not found, trying next candidate")

Behaviour After This Change

Checklist

• No new dependencies
• No breaking API changes
• ruff check + ruff format pass
• Existing test suite: same 4 pre-existing failures unrelated to this change, all others pass