add custom baseurl for cua - #116

Note

Low Risk
Additive SDK fields and serialization only; no changes to auth or existing required CUA parameters.

Overview
Adds optional custom provider base URLs when starting OpenAI CUA agent tasks, so callers can point at proxies or Azure-style endpoints (e.g. https://example.openai.azure.com/openai/v1/) alongside existing custom API keys.

Introduces CuaBaseUrls (Pydantic model and TypedDict) with an optional openai field, wires it into StartCuaTaskParams as base_urls / baseUrls, and re-exports the type from the public models and types packages. Bumps the package version to 1.2.3.

Adds a request-serialization test ensuring dict and model inputs match and emit camelCase baseUrls, plus typecheck fixtures for sync and async client.agents.cua.start.

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