jextract: Fix compilation errors in constrained generic extensions

[sidepelican]

This PR addresses compilation errors occurring in the generated "opener" code for certain constrained generic extensions.

1. Protocol-constrained extensions

extension Foo where Element: SomeProtocol {
    public func something() {} 
}

When an extension has a protocol requirement, the functions were being exported into "opener" code that does not satisfy the protocol constraint, leading to a build failure.

2. Same-type constraints with type parameters on the RHS

public struct Box<Element> {}
public struct Bait {}

extension Box where Bait == Element {
  public func swappedObserveTheBait() {}
}

In cases like where Bait == Element, the generator was incorrectly exporting functions.
Similar to the protocol case, the opener fails to satisfy the same-type requirement when a type parameter is involved on the right-hand side.

I have updated the Swift2JavaVisitor implementation to correctly identify these problematic constraints.