fix: 收紧 streamable fallback jsonization<T> 的 requires

[weinibuliu]

Summary by Sourcery

Bug Fixes:

• 当类型 T 已经匹配内建的 JSON 值、数组、对象，或经工具检测为集合/元组/map/可空类型/variant 等类别时，避免为可流式输出类型选择 jsonization<T>。

Original summary in English

Summary by Sourcery

Bug Fixes:

• Prevent jsonization for streamable types from being selected when T already matches built-in JSON value, array, object, or utility-detected collection/tuple/map/nullable/variant categories.

[sourcery-ai]

Hey - 我在这里给出了一些总体反馈：

• 由于这个回退逻辑现在依赖多个 json::_utils 类型萃取（type traits），建议提取出一个具名 concept（例如 streamable_non_json_type），以便集中约束逻辑，并在这些 traits 在其他地方发生变化时，降低行为不一致的风险。
• 在公共头文件中使用内部的 json::_utils traits 会加深对实现细节的耦合；如果可能的话，更推荐使用对外公开的 traits，或者包一层薄的 concept 封装，从而避免在 API 表面暴露内部的辅助工具。

面向 AI Agents 的提示

Please address the comments from this code review:

## Overall Comments
- Since this fallback now depends on multiple json::_utils type traits, consider extracting a named concept (e.g., `streamable_non_json_type`) to centralize the constraint logic and reduce the risk of divergence if these traits change elsewhere.
- Using internal `json::_utils` traits in a public header can tighten coupling to implementation details; if possible, prefer public-facing traits or a thin wrapper concept to avoid exposing internal helpers in the API surface.

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Original comment in English

Hey - I've left some high level feedback:

• Since this fallback now depends on multiple json::_utils type traits, consider extracting a named concept (e.g., streamable_non_json_type) to centralize the constraint logic and reduce the risk of divergence if these traits change elsewhere.
• Using internal json::_utils traits in a public header can tighten coupling to implementation details; if possible, prefer public-facing traits or a thin wrapper concept to avoid exposing internal helpers in the API surface.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Since this fallback now depends on multiple json::_utils type traits, consider extracting a named concept (e.g., `streamable_non_json_type`) to centralize the constraint logic and reduce the risk of divergence if these traits change elsewhere.
- Using internal `json::_utils` traits in a public header can tighten coupling to implementation details; if possible, prefer public-facing traits or a thin wrapper concept to avoid exposing internal helpers in the API surface.

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[neko-para]

按理说应该能命中is_constructible_v<json::array, T>吧, 先看看为啥没命中?