[Theia AI] Support DeepSeek reasoner (and future models with special "system" prompt requirements

[JonasHelming]

the resoner complains if the first two messages are "user" and does not support developerMessages (nor system). This is currently our default if developer messages are not supported.
To avoid similar issues in the future, I would propose to:

• Introduce a setting "SystemPromptRole" (String) with which you can override the system role (e.g. to "system" or "user")
• Introduce a setting "mergeSystemPrompt" that will merge the system prompt with the first user message

This would kind of make the setting "supportsDeveloperMessage" obsolete.

@planger WDYT?

[planger]

Yeah I guess we could introduce a property as follows instead of the simple flag we had before:

developerMessageSettings: {
  /** Override the role name of the developer message, defaults to 'developer */
  role?: string,
  /** Specify how the developer message is included in the request, defaults to 'putAsFirstMessage' */
  includeInMessages?: 'putAsFirstMessage' | 'mergeWithFirstUserMessage' | 'skip',
}

[JonasHelming]

@planger great suggestion, just one question: Whats the difference between "putAsFirstMessage" and leaving out "includeInMessage"?

[planger]

I think pushAsFirstMessage is what we do now and which I assume will remain to be the default. So when we leave out this property we'll still put the developer message as first message into the message list.

I just thought when we now start to add different strategies here, we should give a name to what we do now and continue to do by default.

[JonasHelming]

Supporting arbitrary strings as roles has quite some implications for unknown benefit. Therefore, i went for:

enum: ['user', 'system', 'developer', 'mergeWithFirstUserMessage', 'skip'],

reasoning is that 'mergeWithFirstUserMessage' and 'skip' do not make sense if the role is also defined.