fix: improve vivado binary handling in argument parser

[taraxacum45e9a]

Fixes a TypeError crash when Vivado isn't on PATH and --vivado isn't explicitly passed. The argparse default is now resolved via shutil.which("vivado") at parse time, and CommandConfiguration.__init__ raises a clear ValueError if no binary could be determined, instead of passing None into Path(...).

Problem

Previously, --vivado defaulted to None, and the fallback lookup (shutil.which("vivado")) happened in __init__. When Vivado wasn't found on PATH, shutil.which returned None, which was then passed directly into Path(...), causing an unhandled TypeError:

TypeError: argument should be a str or an os.PathLike object where __fspath__ returns a str, not 'NoneType'

Fix

• --vivado now defaults to shutil.which("vivado") directly in populate_argument_parser, so the PATH lookup happens once, at argument-parsing time.
• In __init__, if args.vivado is falsy (not provided and not found on PATH), we raise a descriptive ValueError instead of constructing a Path from None.
• The rest of the resolution logic (expanduser().resolve() and the is_file() check) is preserved.

ValueError: Vivado binary not specified and could not be found on PATH.