feat: Add comprehensive callable handler support for closures, static methods, and invokable classes

[CodeWithKyrian]

This PR implements comprehensive support for PHP callables as MCP server handlers, extending beyond the original class/method array format to support closures, static methods, and invokable classes.

What's New

Flexible Handler Types

• Closures: function(string $input): string { return "processed: $input"; }
• Class methods: [ClassName::class, 'methodName'] (resolved via container)
• Static methods: [ClassName::class, 'staticMethod'] (called directly)
• Invokable classes: InvokableClass::class (resolved via container, then __invoke())
• Instance methods: [$instance, 'method'] (called directly on existing instance)

Smart Container Resolution

Updated handler resolution logic to properly distinguish between types that need container resolution vs. direct calls:

1. String handlers (invokable classes) → Resolve from container first
2. Array handlers → Check if class string vs. object instance
3. Other callables → Call directly (closures, static methods, etc.)

Automatic Naming & Caching

• Unique closure names: closure_tool_{spl_object_id} prevents collisions
• Cache safety: Closures are never serialized (logged with warnings)
• Backward compatibility: Existing array format handlers work unchanged

API Changes

Before

->withTool([EmailHandler::class, 'send'], 'send_email')
->withResource([ConfigHandler::class, 'get'], 'config://app')

After (all supported)

// Closures
->withTool(function(string $to, string $msg): string {
    return "Email sent to $to: $msg";
}, 'send_email')

// Static methods  
->withTool([EmailHandler::class, 'validateEmail'], 'validate')

// Invokable classes
->withTool(EmailSender::class, 'send_email') 

// Class methods (original - still works)
->withResource([ConfigHandler::class, 'get'], 'config://app')

Resolves #37