[ty] add cycle handling to BoundMethodType::into_callable_type() (#20369)

## Summary

This looks like it should fix the errors that we've been seeing in sympy
in recent mypy-primer runs.

## Test Plan

I wasn't able to reproduce the sympy failures locally; it looks like
there is probably a dependency on the order in which files are checked.
So I don't have a minimal reproducible example, and wasn't able to add a
test :/ Obviously I would be happier if we could commit a regression
test here, but since the change is straightforward and clearly
desirable, I'm not sure how many hours it's worth trying to track it
down.

Mypy-primer is still failing in CI on this PR, because it fails on the
"old" ty commit already (i.e. on main). But it passes [on a no-op PR
stacked on top of this](https://github.com/astral-sh/ruff/pull/20370),
which strongly suggests this PR fixes the problem.

crates/ty_python_semantic/src/types.rs

@@ -8938,6 +8938,23 @@ fn walk_bound_method_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
     visitor.visit_type(db, method.self_instance(db));
 }
 
+#[allow(clippy::trivially_copy_pass_by_ref)]
+fn into_callable_type_cycle_recover<'db>(
+    _db: &'db dyn Db,
+    _value: &CallableType<'db>,
+    _count: u32,
+    _self: BoundMethodType<'db>,
+) -> salsa::CycleRecoveryAction<CallableType<'db>> {
+    salsa::CycleRecoveryAction::Iterate
+}
+
+fn into_callable_type_cycle_initial<'db>(
+    db: &'db dyn Db,
+    _self: BoundMethodType<'db>,
+) -> CallableType<'db> {
+    CallableType::bottom(db)
+}
+
 #[salsa::tracked]
 impl<'db> BoundMethodType<'db> {
     /// Returns the type that replaces any `typing.Self` annotations in the bound method signature.
@@ -8951,7 +8968,7 @@ impl<'db> BoundMethodType<'db> {
         self_instance
     }
 
-    #[salsa::tracked(heap_size=ruff_memory_usage::heap_size)]
+    #[salsa::tracked(cycle_fn=into_callable_type_cycle_recover, cycle_initial=into_callable_type_cycle_initial, heap_size=ruff_memory_usage::heap_size)]
     pub(crate) fn into_callable_type(self, db: &'db dyn Db) -> CallableType<'db> {
         let function = self.function(db);
         let self_instance = self.typing_self_type(db);
@@ -9089,9 +9106,8 @@ impl<'db> CallableType<'db> {
     ///
     /// Specifically, this represents a callable type with a single signature:
     /// `(*args: object, **kwargs: object) -> Never`.
-    #[cfg(test)]
-    pub(crate) fn bottom(db: &'db dyn Db) -> Type<'db> {
-        Self::single(db, Signature::bottom())
+    pub(crate) fn bottom(db: &'db dyn Db) -> CallableType<'db> {
+        Self::new(db, CallableSignature::bottom(), false)
     }
 
     /// Return a "normalized" version of this `Callable` type.

crates/ty_python_semantic/src/types/property_tests.rs

@@ -158,7 +158,7 @@ mod stable {
     type_property_test!(
         bottom_callable_is_subtype_of_all_callable, db,
         forall types t. t.is_callable_type()
-            => CallableType::bottom(db).is_subtype_of(db, t)
+            => Type::Callable(CallableType::bottom(db)).is_subtype_of(db, t)
     );
 
     // `T` can be assigned to itself.

crates/ty_python_semantic/src/types/signatures.rs

@@ -43,6 +43,10 @@ impl<'db> CallableSignature<'db> {
         }
     }
 
+    pub(crate) fn bottom() -> Self {
+        Self::single(Signature::bottom())
+    }
+
     /// Creates a new `CallableSignature` from an iterator of [`Signature`]s. Returns a
     /// non-callable signature if the iterator is empty.
     pub(crate) fn from_overloads<I>(overloads: I) -> Self