Fix(FlowiseChatGoogleGenerativeAI): Prevent "parts must not be empty" API error in Seq Agents

[toi500]

Problem:
The ChatGoogleGenerativeAI node can fail with Google API errors (requires alternate messages or parts must not be empty) when used in sequential flows like loops or agents. This happens because Flowise appends a final user message which can violate API rules (consecutive user roles or empty parts if "Human Prompt" is empty).

Example Error:

Solution:
Modified the convertBaseMessagesToContent helper function to fix the message list before the API call:

1. Merges Consecutive Roles: A post-processing step now merges consecutive messages if they have the same role and simple text content. This fixes role alternation issues and implicitly removes potentially empty placeholder messages.
2. Adds Safeguard: A final check adds a minimal user message (...) if the list ends in model after merging, ensuring API compliance.

Result:
This makes the node significantly more robust for sequential agents and loops by handling common structural issues caused by Flowise's message handling.

Reproducible Flow:
gemini-seq-agent-bug Agents.json

Also tested successfully with multi-agent setups:

[toi500]

Merged here #4292