Add Completion Support

[pushpak1300]

This PR adds support for the MCP completion utility, allowing servers to provide IDE-like autocompletion suggestions for Prompt arguments and Resource template variables.

Completions are opt-in. MCP servers will not enable this by default. To turn it on, add the capability in your server class:

protected function boot(): void
{
    $this->addCapability(self::CAPABILITY_COMPLETION);
}

How It Works

1. Opt-in per primitive: Prompts and Resources implement SupportsCompletion interface
2. Context-aware: Access current argument values via $context array parameter
3. Lazy resolution: Completion strategies (fromArray, fromEnum, fromCallback) resolve at call time with user input
4. Built-in helpers: Filter completions with CompletionHelper::filterByPrefix()

Completion Strategies

// Direct array (immediate, no filtering)
CompletionResponse::from(['php', 'python', 'javascript']);

// Single value
CompletionResponse::from('php');

// List with prefix filtering (lazy)
CompletionResponse::fromArray(['php', 'python', 'javascript']);

// Enum values (lazy)
CompletionResponse::fromEnum(StatusEnum::class);

// Callback (deferred execution)
CompletionResponse::fromCallback(fn (string $value) => 
    DB::table('tags')->where('name', 'like', "$value%")->pluck('name')->all()
);

// Empty result
CompletionResponse::empty();

Usage Example

class TaskPrompt extends Prompt implements SupportsCompletion
{
    public function complete(string $argument, string $value, array $context): CompletionResponse
    {
        return match ($argument) {
            'assignee' => CompletionResponse::from(['alice', 'bob', 'charlie']),
            'priority' => CompletionResponse::fromEnum(Priority::class),
            'category' => CompletionResponse::fromArray(['backend', 'frontend', 'infra']),
            'tag' => CompletionResponse::fromCallback(
                fn ($v) => DB::table('tags')->where('name', 'like', "$v%")->pluck('name')->all()
            ),
            'status' => $context['priority'] === 'high'
                ? CompletionResponse::make(['in-progress', 'blocked'])
                : CompletionResponse::fromArray(['pending', 'in-progress', 'completed']),
            default => CompletionResponse::empty(),
        };
    }
}

Resource Template Completion

class UserFileResource extends Resource implements HasUriTemplate, SupportsCompletion
{
    public function uriTemplate(): UriTemplate
    {
        return new UriTemplate('file://users/{userId}/files/{fileId}');
    }

    public function complete(string $argument, string $value, array $context): CompletionResponse
    {
        return match ($argument) {
            'userId' => CompletionResponse::fromArray(['user-1', 'user-2']),
            'fileId' => $context['userId'] 
                ? CompletionResponse::fromArray($this->getFilesForUser($context['userId']))
                : CompletionResponse::empty(),
            default => CompletionResponse::empty(),
        };
    }
}

Testing

MyServer::completion(LanguagePrompt::class, 'language', 'py')
    ->assertHasCompletions(['python'])
    ->assertCompletionCount(1);

MyServer::completion(TaskPrompt::class, 'status', '', ['priority' => 'high'])
    ->assertHasCompletions(['in-progress', 'blocked']);