Fix for bug found in Gemini support found during INT tests

[rpkruse]

• The Bug: Google's @google/genai SDK strictly rejects any message in its contents array that isn't explicitly tagged with the user or model role, throwing a 400 INVALID_ARGUMENT error. When the core Memori engine injected recalled facts using a standard { role: 'system' } message, the Axon middleware blindly passed it into the Gemini contents array.
• The Impact: Any attempt to use Memori's recall injection feature with Gemini resulted in an immediate API crash, breaking the integration.
• The Fix: Updated the Axon translation layer (proxy.ts, common.ts, and types.ts) to properly intercept and filter out system messages from the main array. Axon now cleanly extracts and maps the system prompt over to config.systemInstruction, exactly where modern Gemini models expect it. We also completely overhauled the Gemini type definitions along the way