not expand TypeAlias in reveal_type

[asukaminato0721]

part of #1219.

Added TypeAlias::body_type/runtime_value and used them across the solver so we can keep alias names for printing while still acting on their underlying types when needed (attribute access, class bases, isinstance/issubclass, subset checks). Scoped type aliases still behave as TypeAliasType at runtime; legacy aliases now behave like the types they wrap.

Refined isinstance handling so union aliases (e.g., X = B | C) are accepted as class info, and ensured scoped aliases still error.

Allowed type-alias bases when they’re legacy aliases and kept the explicit error for scoped aliases.

[yangdanny97]

Can you elaborate on your proposed fix? I don't think this PR addresses the root cause of #1219 - the type aliases are expanded in untype_opt & the information those types came from an alias is lost.

[asukaminato0721]

at least less noisy now.

[yangdanny97]

I'll import this and see how we can merge it with my in-progress implementation. I'd also like to run it on mypy primer and see what the delta is.

[meta-codesync]

@yangdanny97 has imported this pull request. If you are a Meta employee, you can view this in D86712118.