feat(js/plugins/mcp): support for multipart tools

[ifielker]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request introduces support for multipart tool actions within the MCP (Model Context Protocol) plugin and enhances the Google GenAI plugin's handling of tool responses. Key changes include the addition of a multipart configuration flag for MCP clients and hosts, allowing tools to return structured content such as text and media parts. The toGeminiToolResponse converter in the Google GenAI plugin was updated to process these multipart responses, merging text content and handling media parts for compatibility with Gemini models. Additionally, the MCP SDK was upgraded, and new tests and a test application were added to verify the multipart functionality. I have no feedback to provide as no review comments were submitted.