Fix/24886 extension using defaults

[krrish175-byte]

The compiler was incorrectly reporting "two or more overloaded variants have default arguments" when an extension method with default using clauses coexisted with a non-extension method of the same name.

The issue was that the conflicting defaults check was applied to all methods with the same name, rather than only to methods that actually conflict (i.e., those that match the same static signature).

Changes:

• Move the conflicting defaults check inside the matchesSameStatic condition in Checking.scala
• Add test case demonstrating extension methods with default using clauses working alongside non-extension methods
  This allows valid code patterns where extension methods and regular methods share the same name, since they have different signatures and don't actually overload each other.

Closing issue

closes #24886