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)

README.md

@@ -3,7 +3,7 @@
 [zen_web.webm](https://github.com/user-attachments/assets/851e3911-7f06-47c0-a4ab-a2601236697c)
 
 <div align="center">  
-  <b>🤖 Claude + [Gemini / OpenAI / Grok / OpenRouter / Ollama / Any Model] = Your Ultimate AI Development Team</b>
+  <b>🤖 Claude + [Gemini / OpenAI / Grok / OpenRouter / DIAL / Ollama / Any Model] = Your Ultimate AI Development Team</b>
 </div>
 
 <br/>
@@ -145,6 +145,7 @@ The final implementation resulted in a 26% improvement in JSON parsing performan
 - **Gemini**: Visit [Google AI Studio](https://makersuite.google.com/app/apikey) and generate an API key. For best results with Gemini 2.5 Pro, use a paid API key as the free tier has limited access to the latest models.
 - **OpenAI**: Visit [OpenAI Platform](https://platform.openai.com/api-keys) to get an API key for O3 model access.
 - **X.AI**: Visit [X.AI Console](https://console.x.ai/) to get an API key for GROK model access.
+- **DIAL**: Visit [DIAL Platform](https://dialx.ai/) to get an API key for accessing multiple models through their unified API. DIAL is an open-source AI orchestration platform that provides vendor-agnostic access to models from major providers, open-source community, and self-hosted deployments. [API Documentation](https://dialx.ai/dial_api)
 
 **Option C: Custom API Endpoints (Local models like Ollama, vLLM)**
 [Please see the setup guide](docs/custom_models.md#option-2-custom-api-setup-ollama-vllm-etc). With a custom API you can use:
@@ -154,7 +155,7 @@ The final implementation resulted in a 26% improvement in JSON parsing performan
 - **Text Generation WebUI**: Popular local interface for running models
 - **Any OpenAI-compatible API**: Custom endpoints for your own infrastructure
 
-> **Note:** Using all three options may create ambiguity about which provider / model to use if there is an overlap. 
+> **Note:** Using multiple provider options may create ambiguity about which provider / model to use if there is an overlap. 
 > If all APIs are configured, native APIs will take priority when there is a clash in model name, such as for `gemini` and `o3`.
 > Configure your model aliases and give them unique names in [`conf/custom_models.json`](conf/custom_models.json)
 
@@ -192,6 +193,12 @@ nano .env
 # GEMINI_API_KEY=your-gemini-api-key-here  # For Gemini models
 # OPENAI_API_KEY=your-openai-api-key-here  # For O3 model
 # OPENROUTER_API_KEY=your-openrouter-key  # For OpenRouter (see docs/custom_models.md)
+# DIAL_API_KEY=your-dial-api-key-here      # For DIAL platform
+
+# For DIAL (optional configuration):
+# DIAL_API_HOST=https://core.dialx.ai      # Default DIAL host (optional)
+# DIAL_API_VERSION=2024-12-01-preview      # API version (optional)
+# DIAL_ALLOWED_MODELS=o3,gemini-2.5-pro    # Restrict to specific models (optional)
 
 # For local models (Ollama, vLLM, etc.):
 # CUSTOM_API_URL=http://localhost:11434/v1  # Ollama example
@@ -537,10 +544,11 @@ Configure the Zen MCP Server through environment variables in your `.env` file.
 DEFAULT_MODEL=auto
 GEMINI_API_KEY=your-gemini-key
 OPENAI_API_KEY=your-openai-key
+DIAL_API_KEY=your-dial-key  # Optional: Access to multiple models via DIAL
 ```
 
 **Key Configuration Options:**
-- **API Keys**: Native APIs (Gemini, OpenAI, X.AI), OpenRouter, or Custom endpoints (Ollama, vLLM)
+- **API Keys**: Native APIs (Gemini, OpenAI, X.AI), OpenRouter, DIAL, or Custom endpoints (Ollama, vLLM)
 - **Model Selection**: Auto mode or specific model defaults
 - **Usage Restrictions**: Control which models can be used for cost control
 - **Conversation Settings**: Timeout, turn limits, memory configuration