Fix system message handling to preserve user-configured system prompts

[IAmStoxe]

Problem

Previously, when a preprompt was provided, it would completely replace any user-configured system message instead of being prepended to it.

Solution

• Added explicit detection of existing system messages
• When a system message exists and preprompt is defined, prepend preprompt to the existing content
• When a system message exists and no preprompt is defined, preserve the user's message unchanged
• When no system message exists, create one with either preprompt or empty string
• Maintained handling for models that don't support system roles

Testing

Scenario	User System Prompt	Preprompt	Result
1	❌ No	❌ No	System message with empty content is created
2	❌ No	✅ Yes	System message with preprompt content is created
3	✅ Yes	❌ No	User's system prompt is preserved unchanged
4	✅ Yes	✅ Yes	Preprompt + "\n\n" + User's system prompt

Breaking Changes

None - this change preserves all existing functionality while fixing the prompt handling logic.