Spring-ai Remove unnecessary null checks in AzureOpenAiChatOptions.Builder

This is related to https://github.com/spring-projects/spring-ai/issues/889

This commit addresses a bug where certain fields were being made indirectly mandatory due to Assert.notNull checks in the Builder's with* methods. Specifically:

• Removed Assert.notNull checks from withResponseFormat, withSeed, withLogprobs, withTopLogprobs, and withEnhancements methods.

These checks were causing exceptions in AzureOpenAiChatModel.getDefaultOptions when not all fields were set, leading to failures in methods like ChatClient.create, even when using the AzureOpenAiChatModel constructor with OpenAIClient.

The removal of these checks aligns with the @JsonInclude(Include.NON_NULL) annotation on AzureOpenAiChatOptions, which already ignores null options. This change maintains the intended flexibility while preventing unintended mandatory requirements.

Comment From: markpollack

merged e29d38d987d9aabdb35cf87321b13bb33dbdb871