### What happened?

Clients that send the legacy extended-thinking shape `thinking={"type": "enabled", "budget_tokens": N}` to a 4.6+ Claude model (Opus 4.7 / 4.8, Sonnet 5, Fable 5) through `/chat/completions` have it forwarded to the provider unchanged, and the provider rejects it with a 400. The exact same request sent to `/v1/messages` succeeds, because the messages passthrough already translates the legacy shape up to `thinking={"type": "adaptive"}` + `output_config.effort`

This is the mirror image of the downgrade work in #32867 and #32944. Those handle adaptive -> legacy for pre-4.6 models across `/chat/completions`, `/v1/messages`, and Bedrock Converse. The upgrade direction (legacy enabled -> adaptive for 4.6+ models) exists only on `/v1/messages` (`_translate_legacy_thinking_for_adaptive_model`) and is missing from the whole chat/completions family: `AnthropicConfig.map_openai_params`, Bedrock Converse, Bedrock Invoke chat, Vertex chat, and Responses

Per the Anthropic API, `thinking.type=enabled` with `budget_tokens` is rejected with a 400 on Opus 4.7 / 4.8, Sonnet 5, and Fable 5, and is deprecated on Opus/Sonnet 4.6. So any client still on the budget-based interface th…