feat: DIAL provider implementation (#112)

## Description

This PR implements a new [DIAL](https://dialx.ai/dial_api) (Data & AI Layer) provider for the Zen MCP Server, enabling unified access to multiple AI models through the DIAL API platform. DIAL provides enterprise-grade AI model access with deployment-specific routing similar to Azure OpenAI.

## Changes Made

- [x] Added support of atexit:
  - Ensures automatic cleanup of provider resources (HTTP clients, connection pools) on server shutdown
  - Fixed bug using ModelProviderRegistry.get_available_providers() instead of accessing private _providers
  - Works with SIGTERM/Ctrl+C for graceful shutdown in both development and containerized environments
- [x] Added new DIAL provider (`providers/dial.py`) inheriting from `OpenAICompatibleProvider`
- [x] Updated server.py to register DIAL provider during initialization
- [x] Updated provider registry to include DIAL provider type
- [x] Implemented deployment-specific routing for DIAL's Azure OpenAI-style endpoints
- [x] Implemented performance optimizations:
  - Connection pooling with httpx for better performance
  - Thread-safe client caching with double-check locking pattern
  - Proper resource cleanup with `close()` method
- [x] Added comprehensive unit tests with 16 test cases (`tests/test_dial_provider.py`)
- [x] Added DIAL configuration to `.env.example` with documentation
- [x] Added support for configurable API version via `DIAL_API_VERSION` environment variable
- [x] Added DIAL model restrictions support via `DIAL_ALLOWED_MODELS` environment variable

### Supported DIAL Models:
- OpenAI models: o3, o4-mini (and their dated versions)
- Google models: gemini-2.5-pro, gemini-2.5-flash (including search variant)
- Anthropic models: Claude 4 Opus/Sonnet (with and without thinking mode)

### Environment Variables:
- `DIAL_API_KEY`: Required API key for DIAL authentication
- `DIAL_API_HOST`: Optional base URL (defaults to https://core.dialx.ai)
- `DIAL_API_VERSION`: Optional API version header (defaults to 2025-01-01-preview)
- `DIAL_ALLOWED_MODELS`: Optional comma-separated list of allowed models

### Breaking Changes:
- None

  ### Dependencies:
  - No new dependencies added (uses existing OpenAI SDK with custom routing)

server.py

@@ -19,6 +19,7 @@ as defined by the MCP protocol.
 """
 
 import asyncio
+import atexit
 import logging
 import os
 import sys
@@ -271,6 +272,7 @@ def configure_providers():
     from providers import ModelProviderRegistry
     from providers.base import ProviderType
     from providers.custom import CustomProvider
+    from providers.dial import DIALModelProvider
     from providers.gemini import GeminiModelProvider
     from providers.openai_provider import OpenAIModelProvider
     from providers.openrouter import OpenRouterProvider
@@ -303,6 +305,13 @@ def configure_providers():
         has_native_apis = True
         logger.info("X.AI API key found - GROK models available")
 
+    # Check for DIAL API key
+    dial_key = os.getenv("DIAL_API_KEY")
+    if dial_key and dial_key != "your_dial_api_key_here":
+        valid_providers.append("DIAL")
+        has_native_apis = True
+        logger.info("DIAL API key found - DIAL models available")
+
     # Check for OpenRouter API key
     openrouter_key = os.getenv("OPENROUTER_API_KEY")
     if openrouter_key and openrouter_key != "your_openrouter_api_key_here":
@@ -336,6 +345,8 @@ def configure_providers():
             ModelProviderRegistry.register_provider(ProviderType.OPENAI, OpenAIModelProvider)
         if xai_key and xai_key != "your_xai_api_key_here":
             ModelProviderRegistry.register_provider(ProviderType.XAI, XAIModelProvider)
+        if dial_key and dial_key != "your_dial_api_key_here":
+            ModelProviderRegistry.register_provider(ProviderType.DIAL, DIALModelProvider)
 
     # 2. Custom provider second (for local/private models)
     if has_custom:
@@ -358,6 +369,7 @@ def configure_providers():
             "- GEMINI_API_KEY for Gemini models\n"
             "- OPENAI_API_KEY for OpenAI o3 model\n"
             "- XAI_API_KEY for X.AI GROK models\n"
+            "- DIAL_API_KEY for DIAL models\n"
             "- OPENROUTER_API_KEY for OpenRouter (multiple models)\n"
             "- CUSTOM_API_URL for local models (Ollama, vLLM, etc.)"
         )
@@ -376,6 +388,25 @@ def configure_providers():
     if len(priority_info) > 1:
         logger.info(f"Provider priority: {' → '.join(priority_info)}")
 
+    # Register cleanup function for providers
+    def cleanup_providers():
+        """Clean up all registered providers on shutdown."""
+        try:
+            registry = ModelProviderRegistry()
+            if hasattr(registry, "_initialized_providers"):
+                for provider in list(registry._initialized_providers.items()):
+                    try:
+                        if provider and hasattr(provider, "close"):
+                            provider.close()
+                    except Exception:
+                        # Logger might be closed during shutdown
+                        pass
+        except Exception:
+            # Silently ignore any errors during cleanup
+            pass
+
+    atexit.register(cleanup_providers)
+
     # Check and log model restrictions
     restriction_service = get_restriction_service()
     restrictions = restriction_service.get_restriction_summary()
@@ -390,7 +421,8 @@ def configure_providers():
 
         # Validate restrictions against known models
         provider_instances = {}
-        for provider_type in [ProviderType.GOOGLE, ProviderType.OPENAI]:
+        provider_types_to_validate = [ProviderType.GOOGLE, ProviderType.OPENAI, ProviderType.XAI, ProviderType.DIAL]
+        for provider_type in provider_types_to_validate:
             provider = ModelProviderRegistry.get_provider(provider_type)
             if provider:
                 provider_instances[provider_type] = provider