Add vendor parameter support for chat requests

[LimChihi]

What

• Threaded an optional vendorParameters argument through all chat entry points (sync, async, Combine, streaming) and wrapped streaming bodies as needed.
• Documented usage with a some provider “thinking” example and added regression tests (plus request-capturing tweaks to the mock session) to ensure the extra JSON lands correctly.

Why

Some OpenAI-compatible providers (e.g., Volcengine’s DeepSeek) require custom top-level fields such as thinking.type. The SDK previously ignored any unsupported keys, forcing consumers to fork the models. This change gives developers a standard way to supply provider-specific knobs while keeping the official schema intact.

Affected Areas

• Public chat APIs (OpenAIProtocol, OpenAIAsync, OpenAICombine) and their concrete implementations.
• Request-building internals for chat calls (OpenAI.swift, OpenAI+MakeRequest.swift) plus the new helper types.
• Documentation and tests covering chat usage.