feat: add Couchbase MCP agent sample

[teetangh]

Description

Add a sample agent under contributing/samples/mcp_couchbase_agent/ that demonstrates connecting an ADK agent to Couchbase databases via the couchbase-mcp-server MCP server.

Motivation

Couchbase is a widely used distributed database, and the Couchbase MCP server provides 19+ tools for schema discovery, SQL++ queries, and query performance analysis. Adding a sample agent helps ADK users get started with Couchbase integration quickly, similar to existing samples for other MCP servers (BigQuery, Postgres, Notion, etc.).

Scope

• agent.py — LlmAgent using McpToolset with StdioConnectionParams and uvx couchbase-mcp-server
• README.md — setup instructions, example queries, configuration details, and troubleshooting
• .env — environment variable template for connection credentials
• __init__.py — package init

Related

• MCP server repo: https://github.com/Couchbase-Ecosystem/mcp-server-couchbase
• ADK docs integration page PR: docs: add Couchbase integration page adk-docs#1386

[teetangh]

Closing — the sample code is included inline in the integration docs page (google/adk-docs#1386) per maintainer guidance.