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/streamable_test.go

@@ -143,10 +143,11 @@ func TestStreamableServerTransport(t *testing.T) {
 	initReq := req(1, "initialize", &InitializeParams{})
 	initResp := resp(1, &InitializeResult{
 		Capabilities: &serverCapabilities{
-			Logging:   &loggingCapabilities{},
-			Prompts:   &promptCapabilities{ListChanged: true},
-			Resources: &resourceCapabilities{ListChanged: true},
-			Tools:     &toolCapabilities{ListChanged: true},
+			Completions: &completionCapabilities{},
+			Logging:     &loggingCapabilities{},
+			Prompts:     &promptCapabilities{ListChanged: true},
+			Resources:   &resourceCapabilities{ListChanged: true},
+			Tools:       &toolCapabilities{ListChanged: true},
 		},
 		ProtocolVersion: "2025-03-26",
 		ServerInfo:      &implementation{Name: "testServer", Version: "v1.0.0"},