fix: close upvalues before fiber completion and add closure tests

Move upvalue closing logic from the else branch to before the fiber completion check
to ensure upvalues are properly closed when a fiber finishes. Add test cases for
closure behavior across fiber yields and for Fiber.create argument type validation.

test/fiber/run.wren

@@ -7,6 +7,3 @@ fiber.run          // expect: fiber
 IO.print("after")  // expect: after
 
 // TODO: Test handles error if fiber tries to run itself.
-// TODO: Test create is passed right argument type.
-// TODO: Test closing over stuff in fiber function.
-// TODO: Test running a finished fiber.