Feature/treat nullable fields as required

[HaKIMus]

Q	A
Bug fix?	yes
New feature?	no
Docs?	no
Issues	Fix #585
License	MIT

Context: #585 (comment)

Summary: Nullable fields must be present in the required list of their parent output structure, otherwise platforms (like openai) will reject the call.

Proof of work: #585 (comment)

structured-output-union-types.php output:

file:///~/ai/examples/openai/structured-output-union-types.php#L33 Symfony\AI\Fixtures\StructuredOutput\UnionType\UnionTypeDtofile:///~/ai/fixtures/StructuredOutput/UnionType/UnionTypeDto.php#L14 {#310
  +time: Symfony\AI\Fixtures\StructuredOutput\UnionType\HumanReadableTimeUnionfile:///~/ai/fixtures/StructuredOutput/UnionType/HumanReadableTimeUnion.php#L14 {#316
    +readableTime: "2023-10-07T19:32:10Z"
  }
}

[OskarStark]

That's also my knowledge, all fields needs to be in the required section 👍🏻

[chr-hertel]

Thanks for the fast response and fix!