Provide a clear error when the solver is not instantiated.

[SwissChardLeaf]

Fixes #705. Incorporates discussion from #706.

Hello Diffrax community. This is my first contribution attempt!

Summary

When solver is passed as a class rather than an instance (e.g. diffrax.EulerHeun instead of diffrax.EulerHeun()), diffeqsolve previously failed with obscure errors such as AssertionError or TypeError: functools.partial() argument after ** must be a mapping, not property.
This adds an early _validate_solver check that raises a clear ValueError with a concrete example.

Relation to #706

This implements the approach discussed in #706, incorporating Patrick's review feedback on that PR:

• Guard issubclass with isinstance(solver, type) so non-class inputs do not raise TypeError.
• Use diffrax.Euler() in the error message rather than dfx.Euler().

Design notes (open to feedback)

1. Widened solver annotation
The solver parameter is annotated as AbstractSolver | type[AbstractSolver] so that beartype allows uninstantiated solver classes through to _validate_solver, which then raises the intended ValueError. Without this, beartype rejects the call before our check runs.
2. validated_solver
After validation, the solver is stored in a separate validated_solver: AbstractSolver variable used for the remainder of diffeqsolve. This works well with pyright, even with the widened input type annotation.