fix(jdk-codemodel): resolve receiver type for unqualified method calls

[deer]

Unqualified method calls like foo() previously had no receiverType in the MethodInvocation model node, while qualified calls like this.foo() and obj.foo() did. The implicit receiver for an unqualified call is always the enclosing class instance.

JdkExpressionConverter now tracks the enclosing class type via setEnclosingType(), called from JdkInitializer.visitClass before processTypeElement runs. The unqualified branch of visitMethodInvocation uses this as receiverType instead of leaving it empty.

Tests updated: the existing assertion that unqualified calls have empty receiverType is replaced with two new tests — one verifying resolution to the enclosing class, one verifying inner class calls resolve to the inner class type (not the outer).