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)

providers/registry.py

@@ -118,6 +118,7 @@ class ModelProviderRegistry:
             ProviderType.GOOGLE,  # Direct Gemini access
             ProviderType.OPENAI,  # Direct OpenAI access
             ProviderType.XAI,  # Direct X.AI GROK access
+            ProviderType.DIAL,  # DIAL unified API access
             ProviderType.CUSTOM,  # Local/self-hosted models
             ProviderType.OPENROUTER,  # Catch-all for cloud models
         ]
@@ -237,6 +238,7 @@ class ModelProviderRegistry:
             ProviderType.XAI: "XAI_API_KEY",
             ProviderType.OPENROUTER: "OPENROUTER_API_KEY",
             ProviderType.CUSTOM: "CUSTOM_API_KEY",  # Can be empty for providers that don't need auth
+            ProviderType.DIAL: "DIAL_API_KEY",
         }
 
         env_var = key_mapping.get(provider_type)