feat: Add Multi-Instance Support via Custom Channel Builders

[rwjblue-glean]

Overview

This PR adds support for running multiple SSE server instances with shared state. It introduces a flexible, technology-agnostic approach using channel builders that allow for distributed message delivery across server instances.

Key Features

• Custom Event Queue Builders: Allow customizing how server-sent events are distributed across instances
• Custom Notification Channel Builders: Support distributed notification delivery
• Redis Implementation: Provides a ready-to-use implementation using Redis pub/sub
• No Core Dependencies: Maintains zero external dependencies in the core library
• Comprehensive Tests: Includes tests using a mock broker to verify functionality

Technical Approach

Rather than tightly coupling the library to a specific technology (like Redis), this PR introduces builder functions that can create custom communication channels:

// Create a multi-instance SSE server using Redis
sseServer := server.NewSSEServer(
    mcpServer,
    server.WithBaseURL("https://api.example.com"),
    server.NewRedisEventQueueBuilder(redisClient),
    server.NewRedisNotificationChannelBuilder(redisClient),
)

This approach has several advantages:

• Users can implement different message broker solutions (Redis, RabbitMQ, SQS, etc.)
• The core library remains focused and dependency-free
• Testing is simplified using mock implementations

Implementation Notes

• Channel builders are executed once per SSE session
• The Redis implementation uses pub/sub for real-time message delivery
• Communication channels are bidirectional for both producing and consuming messages
• Tests use a mock broker to verify multi-instance behavior without external dependencies

Example Usage

import (
    "github.com/go-redis/redis/v8"
    "github.com/mark3labs/mcp-go/server"
)

func createMultiInstanceServer() *server.SSEServer {
    // Connect to Redis
    redisClient := redis.NewClient(&redis.Options{
        Addr: "redis:6379",
    })
    
    // Create MCP server
    mcpServer := server.NewMCPServer(
        "my-app", 
        "1.0.0", 
        server.WithToolCapabilities(true),
    )
    
    // Create SSE server with Redis-backed channels
    return server.NewSSEServer(
        mcpServer,
        server.WithBaseURL("https://api.example.com"),
        server.NewRedisEventQueueBuilder(redisClient),
        server.NewRedisNotificationChannelBuilder(redisClient),
    )
}

This solution enables horizontally scaling SSE servers while maintaining session consistency across instances.

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[rwjblue-glean]

currently working on tweaks to the tests