feat(giskard-llm): pass through provider transport config

[Hartorn]

Summary

• Forward caller-provided http_client and default_headers through giskard-llm providers to their native SDK clients.
• Map Google transport config into google.genai.types.HttpOptions, preserving explicit http_options fields over convenience kwargs.
• Document the transport options and add an example for custom CA bundles and default headers.

Notes

• This does not change Azure AI behavior: azure_ai/ still uses AsyncAzureOpenAI, keeps /models stripping, and keeps the existing AZURE_AI_API_VERSION default behavior.
• http_client is caller-owned and is not closed by giskard-llm.
• This intentionally does not add ssl_verify, default_query, max_retries, or Azure AI Foundry OpenAI v1 behavior.

Test plan

• uv run --extra all pytest tests/test_providers.py tests/test_routing.py -q
• uv run ruff check .
• uv run ruff format --check .
• pre-commit during commit:
  • ruff check
  • ruff format
  • basedpyright
  • detect-secrets

[gemini-code-assist]

Code Review

This pull request introduces support for custom transport configurations across all LLM providers, allowing users to pass a caller-owned http_client and default_headers to the underlying SDKs. The changes include updates to the README documentation with usage examples, modifications to the provider classes (OpenAI, Azure, Anthropic, and Google) to handle these new parameters, and the addition of comprehensive unit tests to verify correct forwarding of the transport settings. I have no feedback to provide as there were no review comments.