[ty] Do not bind self to non-positional parameters (#20850)

## Summary

closes astral-sh/ty#1333

## Test Plan

Regression test

Author: sharkdp

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

@@ -473,4 +473,18 @@ class C:
 reveal_type(generic_context(C.f))  # revealed: None
 ```
 
+## Non-positional first parameters
+
+This makes sure that we don't bind `self` if it's not a positional parameter:
+
+```py
+from ty_extensions import CallableTypeOf
+
+class C:
+    def method(*args, **kwargs) -> None: ...
+
+def _(c: CallableTypeOf[C().method]):
+    reveal_type(c)  # revealed: (...) -> None
+```
+
 [self attribute]: https://typing.python.org/en/latest/spec/generics.html#use-in-attribute-annotations

crates/ty_python_semantic/src/types/signatures.rs

@@ -579,7 +579,15 @@ impl<'db> Signature<'db> {
     }
 
     pub(crate) fn bind_self(&self, db: &'db dyn Db, self_type: Option<Type<'db>>) -> Self {
-        let mut parameters = Parameters::new(self.parameters().iter().skip(1).cloned());
+        let mut parameters = self.parameters.iter().cloned().peekable();
+
+        // TODO: Theoretically, for a signature like `f(*args: *tuple[MyClass, int, *tuple[str, ...]])` with
+        // a variadic first parameter, we should also "skip the first parameter" by modifying the tuple type.
+        if parameters.peek().is_some_and(Parameter::is_positional) {
+            parameters.next();
+        }
+
+        let mut parameters = Parameters::new(parameters);
         let mut return_ty = self.return_ty;
         if let Some(self_type) = self_type {
             parameters = parameters.apply_type_mapping_impl(