mcp/examples: add PostgreSQL MCP server example

[gk16]

Add standalone PostgreSQL MCP server example with development environment and tooling. This implementation provides read-only database access, schema introspection, and query capabilities while maintaining clean dependency separation from the main SDK.

Motivation and Context

This is part of #33. This PR adds example of implementing mcp on PostgreSQL from https://modelcontextprotocol.io/examples.
The PR provides a production-ready PostgreSQL MCP server example with proper deployment options, testing, and development tooling.

How Has This Been Tested?

• 17 test cases, including unit tests, integration tests, and edge case handling
• Full docker-compose setup with PostgreSQL database and sample data
• Tested against live PostgreSQL with sample schema (users, products, orders tables)

Breaking Changes

• No Breaking Changes: This is purely additive - a new example in a separate subproject

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Architecture Decisions

• Independent Module: Created examples/postgres/go.mod for this example
• Read-Only Safety: All database operations use read-only transactions to prevent accidental data modification

Files Added

• main.go - PostgreSQL MCP server implementation with schema introspection and query tools
• main_test.go - Comprehensive test suite with mock database testing
• go.mod/go.sum - Independent module definition with local SDK reference
• Dockerfile - Multi-stage build for production deployment
• docker-compose.yml - Complete development environment with PostgreSQL
• init.sql - Sample database schema and data for testing
• Makefile - Build automation and development workflow
• demo.sh - Interactive demonstration script
• README.md - Complete documentation following Python server format
• .env.example - Configuration template
• .gitignore - Proper exclusions for Go and Docker artifacts

Development Experience

• Quick Start: make dev sets up complete environment
• Multiple Run Modes: stdio (default) and HTTP (debugging) modes

MCP Features Implemented

• Resources: Dynamic table discovery with schema information
• Tools: Read-only SQL query execution with transaction safety
• Error Handling: Proper MCP error responses and comprehensive error coverage
• Resource URIs: Standard format postgres://host/table/schema

This implementation serves as a reference for building production MCP servers while maintaining clean project architecture.

[jba]

Thanks for this large contribution. It's a lot of work, and we appreciate that. It's cool that you have a complete implementation, with a Makefile, shell scripts, Dockerfile, and so on.

This is definitely worth living somewhere. I'm just not sure it should live in this repo. There is a lot of code to maintain. What if we get a bug report against the Dockerfile or docker compose config? Or some bug about a runtime failure in a particular installation? I don't know if the maintainers of this repo are up to that sort of commitment.

We're going to think about this and get back to you.

/cc @findleyr @samthanawalla