Disabled thinking no longer sent to adaptive-only models

Patch Changes

• #11073 baa57ba Thanks @archievi! - Stop sending thinking: { type: "disabled" } on ChatAnthropic requests when the user never configured thinking. The disabled value is now only emitted when it is explicitly set, so adaptive-only models (e.g. claude-fable-5) that reject an explicit thinking.type: "disabled" no longer fail with a 400 on a default ChatAnthropic instance.