Add `isPreventDefaultMerge` flag for models options

I think this issue has more to do with #3253.

As far as I understand, in each model, if we set a parameter as null in its Options, it will be pulled from global parameters. Like here:

Prompt buildRequestPrompt(Prompt prompt) {
...
    // Define request options by merging runtime options and default options
    OpenAiChatOptions requestOptions = ModelOptionsUtils.merge(runtimeOptions, this.defaultOptions,
		    OpenAiChatOptions.class);
...

However, if we don't want to pull from global parameters? Like in the issue I specified, the user doesn't want to change the global parameters, but needs to specify null for temperature?

Help me if I understand something wrong, however, as I understand it would be good for us if we add something like a flag to the options, let's say isPreventDefaultMerge and then our options would not merge.

If I understand this correctly and if this makes sense to you, I would be happy to work on it, thanks for your time

Start with the model option merge shown in the issue, especially ModelOptionsUtils.merge and the OpenAiChatOptions request construction. Review related issue #3253 and the existing runtime/default option behavior before deciding the flag semantics. Done should mean the requested null-handling behavior is defined and implemented consistently for model options, with tests covering merge prevention.