Allow assigning ellipsis literal as parameter default value (#14982)

Resolves #14840

## Summary

Usage of ellipsis literal as default argument is allowed in stub files.

## Test Plan

Added mdtest for both python files and stub files.


---------

Co-authored-by: Carl Meyer <carl@oddbird.net>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

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

@@ -122,3 +122,10 @@ class Foo: ...
 x = Foo()
 reveal_type(x)  # revealed: Foo
 ```
+
+## Annotated assignments in stub files are inferred correctly
+
+```pyi path=main.pyi
+x: int = 1
+reveal_type(x) # revealed: Literal[1]
+```