Fix type hint for SymbolicReference.reference property

[emmanuel-ferdman]

PR Summary

This PR fixes mypy errors introduced in v3.1.45 where code like repo.head.ref.checkout() fails type checking even though it works at runtime. The issue is that SymbolicReference.reference was typed to return SymbolicReference, but it actually returns Reference subclasses that have methods like checkout() and tracking_branch(). Changed the return type annotation from SymbolicReference to Reference and added an explicit cast in _get_reference() to match what actually happens at runtime. This is a type-only change with no behavioral impact.

Fixes #2066.

[Byron]

Thanks a lot, looks good to me!

Besides that, we do have mypi type checks, but it seems like they are advisory only and can't fail CI.