mcp: implement completion (#55)

This CL implements a completion handler for the server and a complete
method for the client.

Fixes: #12 
<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
This adds support for completion.

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
Added tests and an example.

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
N/A

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

examples/completion/main.go

@@ -0,0 +1,49 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+// This example demonstrates the minimal code to declare and assign
+// a CompletionHandler to an MCP Server's options.
+func main() {
+	// Define your custom CompletionHandler logic.
+	myCompletionHandler := func(_ context.Context, _ *mcp.ServerSession, params *mcp.CompleteParams) (*mcp.CompleteResult, error) {
+		// In a real application, you'd implement actual completion logic here.
+		// For this example, we return a fixed set of suggestions.
+		var suggestions []string
+		switch params.Ref.Type {
+		case "ref/prompt":
+			suggestions = []string{"suggestion1", "suggestion2", "suggestion3"}
+		case "ref/resource":
+			suggestions = []string{"suggestion4", "suggestion5", "suggestion6"}
+		default:
+			return nil, fmt.Errorf("unrecognized content type %s", params.Ref.Type)
+		}
+
+		return &mcp.CompleteResult{
+			Completion: mcp.CompletionResultDetails{
+				HasMore: false,
+				Total:   len(suggestions),
+				Values:  suggestions,
+			},
+		}, nil
+	}
+
+	// Create the MCP Server instance and assign the handler.
+	// No server running, just showing the configuration.
+	_ = mcp.NewServer("myServer", "v1.0.0", &mcp.ServerOptions{
+		CompletionHandler: myCompletionHandler,
+	})
+
+	log.Println("MCP Server instance created with a CompletionHandler assigned (but not running).")
+	log.Println("This example demonstrates configuration, not live interaction.")
+}