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/client.go

@@ -257,6 +257,7 @@ func (c *Client) AddReceivingMiddleware(middleware ...Middleware[*ClientSession]
 
 // clientMethodInfos maps from the RPC method name to serverMethodInfos.
 var clientMethodInfos = map[string]methodInfo{
+	methodComplete:                  newMethodInfo(sessionMethod((*ClientSession).Complete)),
 	methodPing:                      newMethodInfo(sessionMethod((*ClientSession).ping)),
 	methodListRoots:                 newMethodInfo(clientMethod((*Client).listRoots)),
 	methodCreateMessage:             newMethodInfo(clientMethod((*Client).createMessage)),
@@ -352,6 +353,10 @@ func (cs *ClientSession) ReadResource(ctx context.Context, params *ReadResourceP
 	return handleSend[*ReadResourceResult](ctx, cs, methodReadResource, orZero[Params](params))
 }
 
+func (cs *ClientSession) Complete(ctx context.Context, params *CompleteParams) (*CompleteResult, error) {
+	return handleSend[*CompleteResult](ctx, cs, methodComplete, orZero[Params](params))
+}
+
 func (c *Client) callToolChangedHandler(ctx context.Context, s *ClientSession, params *ToolListChangedParams) (Result, error) {
 	return callNotificationHandler(ctx, c.opts.ToolListChangedHandler, s, params)
 }