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
-->

mcp/server.go

@@ -58,6 +58,8 @@ type ServerOptions struct {
 	RootsListChangedHandler func(context.Context, *ServerSession, *RootsListChangedParams)
 	// If non-nil, called when "notifications/progress" is received.
 	ProgressNotificationHandler func(context.Context, *ServerSession, *ProgressNotificationParams)
+	// If non-nil, called when "completion/complete" is received.
+	CompletionHandler func(context.Context, *ServerSession, *CompleteParams) (*CompleteResult, error)
 	// If non-zero, defines an interval for regular "ping" requests.
 	// If the peer fails to respond to pings originating from the keepalive check,
 	// the session is automatically closed.
@@ -231,6 +233,13 @@ func (s *Server) RemoveResourceTemplates(uriTemplates ...string) {
 		func() bool { return s.resourceTemplates.remove(uriTemplates...) })
 }
 
+func (s *Server) complete(ctx context.Context, ss *ServerSession, params *CompleteParams) (Result, error) {
+	if s.opts.CompletionHandler == nil {
+		return nil, jsonrpc2.ErrMethodNotFound
+	}
+	return s.opts.CompletionHandler(ctx, ss, params)
+}
+
 // changeAndNotify is called when a feature is added or removed.
 // It calls change, which should do the work and report whether a change actually occurred.
 // If there was a change, it notifies a snapshot of the sessions.
@@ -570,6 +579,7 @@ func (s *Server) AddReceivingMiddleware(middleware ...Middleware[*ServerSession]
 
 // serverMethodInfos maps from the RPC method name to serverMethodInfos.
 var serverMethodInfos = map[string]methodInfo{
+	methodComplete:               newMethodInfo(serverMethod((*Server).complete)),
 	methodInitialize:             newMethodInfo(sessionMethod((*ServerSession).initialize)),
 	methodPing:                   newMethodInfo(sessionMethod((*ServerSession).ping)),
 	methodListPrompts:            newMethodInfo(serverMethod((*Server).listPrompts)),
@@ -653,6 +663,7 @@ func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParam
 		// reject unsupported features.
 		ProtocolVersion: version,
 		Capabilities: &serverCapabilities{
+			Completions: &completionCapabilities{},
 			Prompts: &promptCapabilities{
 				ListChanged: true,
 			},