[ty] Add stub mapping support to signature help (#19570)
This PR improves the "signature help" language server feature in two
ways:
1. It adds support for the recently-introduced "stub mapper" which maps
symbol declarations within stubs to their implementation counterparts.
This allows the signature help to display docstrings from the original
implementation.
2. It incorporates a more robust fix to a bug that was addressed in a
[previous PR](https://github.com/astral-sh/ruff/pull/19542). It also
adds more comprehensive tests to cover this case.
Co-authored-by: UnboundVariable <unbound@gmail.com>
