Add troubleshooting guide to MCP servers for Gemini CLI

[ghchinoy]

Users report sometimes getting this error on Gemini CLI startup

Error connecting to MCP server 'veo': failed to start or connect to MCP server 'veo' {"command":"mcp-veo-go","trust":true}; 
Error: spawn mcp-veo-go ENOENT
Error connecting to MCP server 'imagen': failed to start or connect to MCP server 'imagen' {"command":"mcp-imagen-go","trust":true}; 
Error: spawn mcp-imagen-go ENOENT
Error connecting to MCP server 'chirp3-hd': failed to start or connect to MCP server 'chirp3-hd' {"command":"mcp-chirp3-go","trust":true}; 
Error: spawn mcp-chirp3-go ENOENT
Error connecting to MCP server 'lyria': failed to start or connect to MCP server 'lyria' {"command":"mcp-lyria-go","trust":true}; 
Error: spawn mcp-lyria-go ENOENT
Error connecting to MCP server 'avtool': failed to start or connect to MCP server 'avtool' {"command":"mcp-avtool-go","trust":true}; 
Error: spawn mcp-avtool-go ENOENT

This is most likely a missing PATH issue, i.e. the genmedia mcp servers are not in the current terminal's path.

This is rectified via export PATH=$PATH:~/go/bin

Add this info to a "Troubleshooting" section here https://github.com/GoogleCloudPlatform/vertex-ai-creative-studio/tree/main/experiments/mcp-genmedia/sample-agents/geminicli