Raise InvalidEntryPoint for invalid entry points

[Himanshuagrawal4]

Summary

• add an InvalidEntryPoint exception for invalid entry_points.txt entries
• raise the clearer exception from parse_entrypoints() instead of surfacing a bare AssertionError
• add regression coverage for direct parsing and the install() path

Rationale

When a wheel contains an invalid entry point target, parse_entrypoints() currently fails with AssertionError, which does not tell users which entry was invalid or why parsing failed. The new exception keeps the failure explicit while carrying the section, name, value, and reason so installers and callers can present a useful message.

Fixes #198.

Testing

• PYTHONPATH=src python -m pytest