feat(genkit_anthropic): add beta/stable API selection and native structured outputs

[adesinah]

The plugin always called stable messages.create, so no beta-gated feature was reachable, and structured output was faked with a synthetic return_output tool forced via tool_choice - which Anthropic rejects alongside extended thinking.

The PR adds apiVersion ('stable' | 'beta') on both the plugin and per-request config, with the request winning and stable as the default, plus a betas override for features the curated list does not know yet. Beta requests send the anthropic-beta header on the unary and streaming paths alike. Models on Anthropic's Structured Outputs list now send the schema natively via output_config.format, so structured output composes with manual thinking. Other models keep the forced-tool fallback, which now reports INVALID_ARGUMENT when combined with manual thinking rather than letting the API reject the request.

Also fixes a caller-supplied toolChoice silently unforcing return_output.

Closes #356

[gemini-code-assist]

Code Review

This pull request introduces support for Anthropic's stable and beta APIs, allowing users to opt into beta-gated features via apiVersion and betas options. It also implements native structured output support for capable models by mapping schemas directly to the output_config.format parameter, while maintaining a forced-tool fallback for older or uncurated models. The reviewer suggested an improvement to _toAnthropicSchema to automatically inject 'type': 'object' when properties or required are present but type is omitted, ensuring better robustness and consistency with the fallback path.