[ty] Fix false positive LSP violation for positional-only params with defaults
When a subclass method has a positional-only parameter with a default value
and the base class overload has only keyword parameters (`**kwargs`), the
override should be valid because callers don't need to provide the positional
argument.
Previously, the signature comparison logic at `signatures.rs:1618-1624`
would unconditionally reject any unmatched positional-only parameter when
comparing against a base signature that only had keyword parameters. This
fix adds a check for whether the positional-only parameter has a default
value, and only rejects it if it doesn't.
Fixes astral-sh/ty#2693
https://claude.ai/code/session_011QG3kSTsewc8vNsUTtVL9G