api: Use Huma to generate swagger docs and parse JSON automatically (#262)

## Motivation and Context

Currently there's a lot of unsafe manual JSON unmarshalling/marshalling,
and we have to manually keep the openapi schema in sync.

This PR migrates the API to
[Huma](https://github.com/danielgtaylor/huma), which appears like a
popular, well-maintained way to build APIs in Golang. I'm not a Golang
expert, but seemed highly recommended by Reddit + Claude and has a
decent star count. It provides automatic openapi spec generation and
docs hosting, JSON parsing, as well as the ability to add auth
middlewares etc.

## How Has This Been Tested?

Running locally, and poking at the endpoints

## Breaking Changes

Docs are served at `/docs` rather than `/v0/swagger/index.html` now

## 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)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] 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
-->

internal/api/handlers/v0/ping_test.go

@@ -0,0 +1,35 @@
+package v0_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/danielgtaylor/huma/v2"
+	"github.com/danielgtaylor/huma/v2/adapters/humago"
+	v0 "github.com/modelcontextprotocol/registry/internal/api/handlers/v0"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPingEndpoint(t *testing.T) {
+	// Create a new test API
+	mux := http.NewServeMux()
+	api := humago.New(mux, huma.DefaultConfig("Test API", "1.0.0"))
+	
+	// Register the ping endpoint
+	v0.RegisterPingEndpoint(api)
+
+	// Create a test request
+	req := httptest.NewRequest(http.MethodGet, "/v0/ping", nil)
+	w := httptest.NewRecorder()
+
+	// Serve the request
+	mux.ServeHTTP(w, req)
+
+	// Check the status code
+	assert.Equal(t, http.StatusOK, w.Code)
+
+	// Check the response body contains pong
+	body := w.Body.String()
+	assert.Contains(t, body, `"pong":true`)
+}