feat: Add support for different content types in Assistants API and other fixes

[davidmigloz]

Adds support for different content types. E.g.

CreateMessageRequestContent.text: for a string content

final res = await client.createThreadMessage(
  threadId: threadId,
  request: const CreateMessageRequest(
    role: MessageRole.user,
    content: CreateMessageRequestContent.text(
      'I need to solve the equation `3x + 11 = 14`. Can you help me?',
    ),
  ),
);

CreateMessageRequestContent.parts: for content that includes text, image URLs or image files

final res = await client.createThreadMessage(
  threadId: threadId,
  request: CreateMessageRequest(
    role: CreateMessageRequestRole.user,
    content: CreateMessageRequestContent.parts([
      MessageContent.text(
        text: MessageContentText(
          value: 'Some text...', 
        ),
      ),
      MessageContent.imageFile(
        imageFile: MessageContentImageFile(fileId: 'file-abc123'),
      ),
      MessageContent.imageUrl(
        imageUrl: MessageContentImageUrl(
          url: 'https://example.com/image.jpg',
        ),
      ),
    ]),
  ),
);

This PR also fixes:

• Client not handling vector stores with null names
  • Resolves OpenAI createVectorStore throw an exception #409
  • Resolves OpenAI listVectorStoreFiles throw an error if the list is empty #410
• Error deserializing RunStepDeltaStepDetailsToolCalls
  • Ref: Migrate OpenAI Assistants API to v2 #401 (comment)