[ty] Return Option<TupleType> from infer_tuple_type_expression (#19735)

## Summary

This PR reduces the virality of some of the `Todo` types in
`infer_tuple_type_expression`. Rather than inferring `Todo`, we instead
infer `tuple[Todo, ...]`. This reflects the fact that whatever the
contents of the slice in a `tuple[]` type expression, we would always
infer some kind of tuple type as the result of the type expression. Any
tuple type should be assignable to `tuple[Todo, ...]`, so this shouldn't
introduce any new false positives; this can be seen in the ecosystem
report.

As a result of the change, we are now able to enforce in the signature
of `Type::infer_tuple_type_expression` that it returns an
`Option<TupleType<'db>>`, which is more strongly typed and expresses
clearly the invariant that a tuple type expression should always be
inferred as a `tuple` type. To enable this, it was necessary to refactor
several `TupleType` constructors in `tuple.rs` so that they return
`Option<TupleType>` rather than `Type`; this means that callers of these
constructor functions are now free to either propagate the
`Option<TupleType<'db>>` or convert it to a `Type<'db>`.

## Test Plan

Mdtests updated.

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -59,7 +59,7 @@ reveal_type(d)  # revealed: tuple[tuple[str, str], tuple[int, int]]
 reveal_type(e)  # revealed: tuple[str, ...]
 
 reveal_type(f)  # revealed: tuple[str, *tuple[int, ...], bytes]
-reveal_type(g)  # revealed: @Todo(PEP 646)
+reveal_type(g)  # revealed: tuple[@Todo(PEP 646), ...]
 
 reveal_type(h)  # revealed: tuple[list[int], list[int]]
 reveal_type(i)  # revealed: tuple[str | int, str | int]