bug: Gemini provider drops system-message semantics

[JNK234]

Problem

The Gemini provider currently converts every non-assistant role into user, so system messages are not preserved as system instructions.

Current behavior

GeminiProvider.createProviderRequest() maps roles like this:

• assistant -> model
• everything else -> user

That means any system message created by:

• llm:chat-with-template
• manually injected history via llm:set-history
• future prompt-steering features

is downgraded to a plain user message before it reaches Gemini.

Why this matters

System prompts are part of the extension’s advertised interaction model. Other providers preserve them, but Gemini currently does not. That creates provider-specific behavior differences for the same NetLogo code:

• template steering becomes weaker or inconsistent on Gemini
• system persona/history instructions are interpreted as ordinary user text
• cross-provider experiments are no longer comparable

Affected code

• src/main/providers/GeminiProvider.scala

Suggested fix directions

1. Handle system prompts using Gemini’s dedicated instruction mechanism instead of flattening them into user messages.
2. If Gemini requires a different request shape, split system instructions from conversational contents before serialization.
3. Add provider-level tests covering a request with system, user, and assistant messages so the serialized request can be validated without live API calls.

[JNK234]

Rechecked against ff094c6 on main. Still open. GeminiProvider.createProviderRequest() continues to map assistant -> model and everything else to user, so system prompts are still flattened instead of being sent through Gemini's dedicated instruction mechanism. This matters more now that template chat is otherwise behaving correctly on current main.

[JNK234]

Rechecked after pulling 3d1283f on main. Still open. The recent Gemini changes improved response parsing, but createProviderRequest() still maps assistant -> model and everything else to user, so system prompts are still flattened instead of being sent via Gemini's instruction mechanism.