fix: Wrap non-JSON tool responses for Gemini compatibility

[azanux]

#1391

Summary

Google GenAI (Gemini) requires tool responses (FunctionResponse.response) to be valid JSON objects. When a tool returns a plain text string, Spring AI's GoogleGenAiChatModel.parseJsonToMap() either:

1. Crashes with a JsonParseException - for MatryoshkaTool responses that return plain text like "Enabled 4 tools: ..."
2. Silently drops the content - for RAG/search tools returning plain text results, causing the LLM to hallucinate

The same tools work correctly with OpenAI, which accepts plain text in tool response content fields.

Root cause

All tool results flow through messageConverters.kt before being sent to the LLM. The textContent was passed as-is to ToolResponseMessage.ToolResponse.responseData without ensuring it's valid JSON.

Most tools worked "by luck" because MethodTool.convertResult() serializes non-String return types to JSON via Jackson only tools returning a raw String (or implementing Tool directly like MatryoshkaTool) were affected.

Fix

• messageConverters.kt: Added ensureJson() extension function that wraps non-JSON tool response text in {"result": "..."} before sending to the LLM.
• JSON objects and arrays are preserved as-is. This acts as a safety net at the single conversion point all tools pass through.
• MatryoshkaTool.kt: Changed SimpleMatryoshkaTool.call() and SelectableMatryoshkaTool.call() to return structured JSON ({"enabled_tools_count": N, "enabled_tools": [...]}) instead of plain text, via a shared enabledToolsJson() helper.

Test plan

• MessageConversionTest: new tests covering plain text wrapping, JSON object/array preservation, and whitespace handling
• MatryoshkaToolTest: All assertions updated to verify JSON format responses
• Manual verification with https://github.com/embabel/ragbot + Google GenAI to confirm no more JsonParseException and no hallucination