Investigate removing @Nullable options in Prompt and ImagePrompt

[markpollack]

Should be doable, just need to look at the impact. The revert of doing this is here

#2950

as it broke some basic prompt behavior in ChatClient.

System.out.println("\n>>> ASSISTANT: " + chatClient.prompt(userInput).call().content());

from https://github.com/spring-projects/spring-ai-examples/blob/main/model-context-protocol/client-starter/starter-default-client/src/main/java/org/springframework/ai/mcp/samples/client/Application.java

[markpollack]

Decided to leave them in as the the impact is very large. Can look into it post 1.0 GA