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/closure.wren

@@ -0,0 +1,25 @@
+var fiber
+var closure
+
+{
+  var a = "before"
+  fiber = Fiber.create(fn {
+    IO.print(a)
+    Fiber.yield
+    a = "after"
+    Fiber.yield
+    IO.print(a)
+    a = "final"
+  })
+
+  closure = fn {
+    IO.print(a)
+  }
+}
+
+fiber.run    // expect: before
+closure.call // expect: before
+fiber.run
+closure.call // expect: after
+fiber.run    // expect: after
+closure.call // expect: final

test/fiber/create_wrong_arg_type.wren

@@ -0,0 +1 @@
+var fiber = Fiber.create("not fn") // expect runtime error: Argument must be a function.

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.