fix: Support types.UnionType in schemas

[booxter]

What does this PR do?

Since Python 3.10, unions can be expressed as type1 | type2. Sadly,
while this is functionally equivalent to Union[type1, type2], the type
of the expression is different (types.UnionType, not typing.Union).

We should handle both in schemas.

Test Plan

Switch a schema type from Union to | and confirm the generator doesn't crash with:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/generate.py", line 91, in <module>
    fire.Fire(main)
  File "/Users/ihrachys/.cache/uv/archive-v0/FBgkcwcN-PaJ0NAur__7J/lib/python3.11/site-packages/fire/core.py", line 135, in Fire
    component_trace = _Fire(component, args, parsed_flag_args, context, name)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/.cache/uv/archive-v0/FBgkcwcN-PaJ0NAur__7J/lib/python3.11/site-packages/fire/core.py", line 468, in _Fire
    component, remaining_args = _CallAndUpdateTrace(
                                ^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/.cache/uv/archive-v0/FBgkcwcN-PaJ0NAur__7J/lib/python3.11/site-packages/fire/core.py", line 684, in _CallAndUpdateTrace
    component = fn(*varargs, **kwargs)
                ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/generate.py", line 55, in main
    spec = Specification(
           ^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/pyopenapi/utility.py", line 30, in __init__
    self.document = generator.generate()
                    ^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/pyopenapi/generator.py", line 782, in generate
    operation = self._build_operation(op)
                ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/pyopenapi/generator.py", line 648, in _build_operation
    "application/json": builder.build_media_type(
                        ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/pyopenapi/generator.py", line 221, in build_media_type
    schema = self.schema_builder.classdef_to_ref(item_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/pyopenapi/generator.py", line 135, in classdef_to_ref
    type_schema = self.classdef_to_schema(typ)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/docs/openapi_generator/pyopenapi/generator.py", line 116, in classdef_to_schema
    type_schema, type_definitions = self.schema_generator.classdef_to_schema(typ)
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/llama_stack/strong_typing/schema.py", line 607, in classdef_to_schema
    types_defined[sub_name] = self._type_to_schema_with_lookup(sub_type)
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/llama_stack/strong_typing/schema.py", line 564, in _type_to_schema_with_lookup
    type_schema = self.type_to_schema(data_type, force_expand=True)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/llama_stack/strong_typing/schema.py", line 320, in type_to_schema
    return self._type_to_schema(data_type, force_expand, json_schema_extra) | common_info
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/llama_stack/strong_typing/schema.py", line 487, in _type_to_schema
    property_docstrings = get_class_property_docstrings(typ, self.options.property_description_fun)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ihrachys/src/llama-stack/llama_stack/strong_typing/schema.py", line 94, in get_class_property_docstrings
    for base in inspect.getmro(data_type):
                ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/w2wykgpkzidnnr6cpw8wf94ghb0p8big-python3-3.11.11/lib/python3.11/inspect.py", line 731, in getmro
    return cls.__mro__
           ^^^^^^^^^^^
AttributeError: 'types.UnionType' object has no attribute '__mro__'. Did you mean: '__or__'?

[booxter]

@ehhuang FYI

[ehhuang]

nice!