[ty] fix unpacking a type alias with detailed tuple spec (#19981)

## Summary

Fixes https://github.com/astral-sh/ty/issues/1046

We special-case iteration of certain types because they may have a more
detailed tuple-spec. Now that type aliases are a distinct type variant,
we need to handle them as well.

I don't love that `Type::TypeAlias` means we have to remember to add a
case for it basically anywhere we are special-casing a certain kind of
type, but at the moment I don't have a better plan. It's another
argument for avoiding fallback cases in `Type` matches, which we usually
prefer; I've updated this match statement to be comprehensive.

## Test Plan

Added mdtest.

crates/ty_python_semantic/resources/mdtest/pep695_type_aliases.md

@@ -64,6 +64,17 @@ x: MyIntOrStr = 1
 y: MyIntOrStr = None
 ```
 
+## Unpacking from a type alias
+
+```py
+type T = tuple[int, str]
+
+def f(x: T):
+    a, b = x
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: str
+```
+
 ## Generic type aliases
 
 ```py

crates/ty_python_semantic/src/types.rs

@@ -4866,7 +4866,39 @@ impl<'db> Type<'db> {
                 // diagnostic in unreachable code.
                 return Ok(Cow::Owned(TupleSpec::homogeneous(Type::unknown())));
             }
-            _ => {}
+            Type::TypeAlias(alias) => {
+                return alias.value_type(db).try_iterate_with_mode(db, mode);
+            }
+            Type::Dynamic(_)
+            | Type::FunctionLiteral(_)
+            | Type::GenericAlias(_)
+            | Type::BoundMethod(_)
+            | Type::MethodWrapper(_)
+            | Type::WrapperDescriptor(_)
+            | Type::DataclassDecorator(_)
+            | Type::DataclassTransformer(_)
+            | Type::Callable(_)
+            | Type::ModuleLiteral(_)
+            | Type::ClassLiteral(_)
+            | Type::SubclassOf(_)
+            | Type::ProtocolInstance(_)
+            | Type::SpecialForm(_)
+            | Type::KnownInstance(_)
+            | Type::PropertyInstance(_)
+            | Type::Union(_)
+            | Type::Intersection(_)
+            | Type::AlwaysTruthy
+            | Type::AlwaysFalsy
+            | Type::IntLiteral(_)
+            | Type::BooleanLiteral(_)
+            | Type::EnumLiteral(_)
+            | Type::LiteralString
+            | Type::BytesLiteral(_)
+            | Type::TypeVar(_)
+            | Type::NonInferableTypeVar(_)
+            | Type::BoundSuper(_)
+            | Type::TypeIs(_)
+            | Type::TypedDict(_) => {}
         }
 
         let try_call_dunder_getitem = || {