Keep declaration order in Foundation Models schemas

[akbashev]

Foundation Models generates an object's properties in the order its schema lists them. The conversion went through JSONValue and JSONSchema, whose decoding loses key order, so properties reached the model shuffled. A container's items could come before what the container is, and generation ran away nesting.

The conversion now goes straight from GenerationSchema's nodes, which keep propertyOrder.

[mattt]

Thanks, @akbashev. Converting straight from the nodes makes sense to me.

One catch: with this change, the JSONSchema path that #332 fixes isn't used anymore, and the .ref case in convertToDynamicSchema(_ node:) drops name the same way the old code did. So if both PRs merge, #332's fix is lost — and there's no conflict to warn us. Could you carry that fix over here (keep a named schema whose one choice is the reference), with a test at the GenerationSchema level?

Two smaller things:

• Int(minimum) truncates a fractional minimum and traps on a non-finite or very large one.
• appearInOrder doesn't look like it's used anywhere.

[akbashev]

Thanks, @mattt.

• Keep the name of a schema that only refers to another #332's fix is carried over: in the node conversion, a named definition that's only a reference keeps its name. convertNamedOneChoiceReferenceFromGenerationSchema builds it from a GenerationSchema and fails without the fix (undefinedReferences ["Action"]).
• Integer bounds now round inward to whole numbers. A bound no Int can hold sets no guide, instead of truncating or trapping. The same trap was also in the stand-in value for an integer (Int(num.minimum ?? 0)), so that's fixed too, with tests.
• appearInOrder is removed.

One question: the JSONSchema path is now used only by its own tests. Would you like me to remove it here, or in a follow-up?

[akbashev]

Merged main in: it conflicted with #332's test in the same file. Both tests are kept.

[mattt]

@akbashev Let's do that in a follow-up, so this PR stays focused on the ordering.