Add custom Contract service injection support for Platform configurations

[Copilot]

This PR implements support for injecting custom Contract services into Platform instances via bundle configuration, enabling platform-specific customization of request/response handling, authentication, and other contract logic.

Changes

Core Implementation

• Added ContractInterface - New interface that custom contract services must implement for type safety
• Enhanced Configuration - Added optional contract key to all platform configurations (anthropic, azure, google, openai, mistral, openrouter)
• Updated Extension Logic - Modified platform creation to inject contract service references when provided
• Autoconfiguration Support - Registered ContractInterface for automatic service discovery

Configuration Example

llm_chain:
  platforms:
    openai:
      api_key: '%env(OPENAI_API_KEY)%'
      contract: '@my_custom_openai_contract_service'
    azure_openai:
      api_key: '%env(AZURE_OPENAI_API_KEY)%'
      contract: '@my_custom_azure_contract_service'

Key Features

• ✅ Optional and Backward Compatible - Contract injection defaults to null, existing configurations work unchanged
• ✅ Type Safe - Custom contracts must implement ContractInterface
• ✅ All Platform Types Supported - Works with OpenAI, Azure, Google, Anthropic, Mistral, and OpenRouter
• ✅ Symfony Service Integration - Uses standard service reference patterns (@service_name)
• ✅ Minimal Changes - Only additions, no modifications to existing functionality

Usage

1. Create a custom contract service implementing ContractInterface
2. Register it in Symfony (autoconfigured with llm_chain.platform.contract tag)
3. Reference it in platform configuration using contract: '@your_service_name'

Testing

• Comprehensive unit tests for configuration and extension logic
• Integration tests validating end-to-end workflow
• Backward compatibility tests ensuring existing configs continue working
• Example implementation demonstrating real-world usage

Documentation

• Updated README with configuration examples
• Added complete section on creating and using custom contract services
• Included integration examples for different platforms

Fixes #95.

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