[ty] Fix standalone expression type retrieval in presence of cycles (astral-sh#17849)

## Summary

When entering an `infer_expression_types` cycle from
`TypeInferenceBuilder::infer_standalone_expression`, we might get back a
`TypeInference::cycle_fallback(…)` that doesn't actually contain any new
types, but instead it contains a `cycle_fallback_type` which is set to
`Some(Type::Never)`. When calling `self.extend(…)`, we therefore don't
really pull in a type for the expression we're interested in. This
caused us to panic if we tried to call `self.expression_type(…)` after
`self.extend(…)`.

The proposed fix here is to retrieve that type from the nested
`TypeInferenceBuilder` directly, which will correctly fall back to
`cycle_fallback_type`.

## Details

I minimized the second example from astral-sh#17792 a bit further and used this
example for debugging:

```py
from __future__ import annotations

class C: ...

def f(arg: C):
    pass

x, _ = f(1)

assert x
```

This is self-referential because when we check the assignment statement
`x, _ = f(1)`, we need to look up the signature of `f`. Since evaluation
of annotations is deferred, we look up the public type of `C` for the
`arg` parameter. The public use of `C` is visibility-constraint by "`x`"
via the `assert` statement. While evaluating this constraint, we need to
look up the type of `x`, which in turn leads us back to the `x, _ =
f(1)` definition.

The reason why this only showed up in the relatively peculiar case with
unpack assignments is the code here:


https://github.com/astral-sh/ruff/blob/78b4c3ccf1d6cb10613671ccec09cafba0d1de72/crates/ty_python_semantic/src/types/infer.rs#L2709-L2718

For a non-unpack assignment like `x = f(1)`, we would not try to infer
the right-hand side eagerly. Instead, we would enter a
`infer_definition_types` cycle that handles the situation correctly. For
unpack assignments, however, we try to infer the type of `value`
(`f(1)`) and therefore enter the cycle via `standalone_expression_type
=> infer_expression_type`.

closes astral-sh#17792 

## Test Plan

* New regression test
* Made sure that we can now run successfully on scipy => see astral-sh#17850

Author: sharkdp

crates/ty_project/resources/test/corpus/88_regression_issue_17792.py

@@ -0,0 +1,15 @@
+# Regression test for https://github.com/astral-sh/ruff/issues/17792
+
+from __future__ import annotations
+
+
+class C: ...
+
+
+def f(arg: C):
+    pass
+
+
+x, _ = f(1)
+
+assert x

crates/ty_python_semantic/src/types/infer.rs

@@ -4009,7 +4009,12 @@ impl<'db> TypeInferenceBuilder<'db> {
         let standalone_expression = self.index.expression(expression);
         let types = infer_expression_types(self.db(), standalone_expression);
         self.extend(types);
-        self.expression_type(expression)
+
+        // Instead of calling `self.expression_type(expr)` after extending here, we get
+        // the result from `types` directly because we might be in cycle recovery where
+        // `types.cycle_fallback_type` is `Some(fallback_ty)`, which we can retrieve by
+        // using `expression_type` on `types`:
+        types.expression_type(expression.scoped_expression_id(self.db(), self.scope()))
     }
 
     fn infer_expression_impl(&mut self, expression: &ast::Expr) -> Type<'db> {