fix: retry with nudge when LLM returns empty response after tool use

[elseasama]

Summary

• Some models (e.g. kimi-k2.5 via NVIDIA API) occasionally return empty content after processing tool call results, causing the bot to reply with "I've completed processing but have no response to give."
• When the LLM returns empty content after a tool use iteration (iteration > 1), the agent now appends a nudge message and retries, giving the model another chance to generate a proper response
• The retry only happens once and respects maxIterations to avoid infinite loops

Test plan

• Send a message that triggers a tool call (e.g. web search) and verify the bot responds with actual content
• Verify that direct answers (no tool calls) are unaffected
• Verify that the nudge retry only fires once per request (check logs for "Empty response after tool use, retrying with nudge")