Allow custom `Contract` service injection in `Platform` class via bundle config

[llupa]

Currently, the Platform class does not provide a way to inject or reference a custom Contract service. This limitation makes it impossible to use different contract implementations per platform when configuring the bundle in Symfony.

I would like to be able to define a custom Symfony service as the contract per platform definition in the bundle configuration.

Example YAML configuration:

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'

---

Acceptance Criteria

• The Contract class should implement a ContractInterface
• Add support for a contract key under each platform in the bundle configuration.
• The contract key when not set must default to null
• The contract key must accept a reference to a Symfony service implementing the required ContractInterface.
• The configured contract service is injected into the corresponding Platform instance at runtime.
• Update documentation and example configs.

[chr-hertel]

Thanks @llupa, let's do it!

We should also add an example how to configure the contract service with using its create method via factory and the custom normalizers as arguments.