[FEATURE]: Allow provider-native options in stateless generate.text

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Describe the enhancement you want to request

Following #39243, V2 now has a stateless generate.text API, but its input only exposes model and prompt. Plugins sometimes need per-request provider controls, such as an output token limit, without creating a session.

Allow callers to pass an optional provider-native JSON body:

await ctx.generate.text({
  model: { providerID: "anthropic", id: "claude-sonnet-4-6" },
  prompt: "Review this action.",
  body: { max_tokens: 2_000 },
})

The body could use the existing raw request overlay after protocol lowering. Keeping it generic avoids adding a new API field for every provider option, while calls that omit it retain the current behavior.