Preserve subscription serializer round trips

[ydah]

This follows up on #5722 with two additional cases where GraphQL::Subscriptions::Serialize.dump and .load don't round-trip their input.

First, arrays are treated as GlobalID arrays based only on their first element:

value = [GlobalIDUser.new("a"), 1, "two"]

GraphQL::Subscriptions::Serialize.load(
  GraphQL::Subscriptions::Serialize.dump(value)
)
# => TypeError: no implicit conversion of String into Integer

The loader attempts to read __gid__ from every remaining element. Other mixed values may produce nil IDs and cause GlobalID::Locator.locate_many to return incorrect results or fail.

Second, Symbol and String keys with the same name are collapsed when keys are converted to strings:

value = { a: 1, "a" => 2 }

dumped = GraphQL::Subscriptions::Serialize.dump(value)
# => {"a":2,"__sym_keys__":["a"]}

GraphQL::Subscriptions::Serialize.load(dumped)
# => { a: 2 }

This PR only uses the batched GlobalID::Locator.locate_many path when every array element is a GlobalID wrapper. Mixed arrays use the existing recursive loading path, while homogeneous GlobalID arrays retain batched lookup.

For Hash key collisions, Symbol-keyed values are stored separately in the existing __sym_keys__ metadata. Hashes without collisions retain their existing serialized format.