fix(vision): preserve native image blocks during handoff

[christoph-hess-itemis]

Summary

• preserve original ImageBlocks for exact request routes that declare image input
• rewrite images to durable describe_image references only for text-only or undeclared routes
• make handoff re-entry request-local and teardown/reconfiguration safe
• keep the auxiliary vision route’s capability metadata honest
• add focused Node regression coverage and bilingual documentation

Root cause

The v0.6.4 handoff listener treated every non-auxiliary route as text-only. It never resolved the exact request route’s real modalities, so native vision models were downgraded to text references. Its global re-entry flag and resolver replacement also made concurrent streams and teardown fragile, while inflating the configured auxiliary route could hide an invalid vision selection.

Validation

• npm run typecheck
• npm test (14/14)
• npm pack --dry-run --json --cache .npm-cache
• git diff --check

Scope / follow-up

This remains a plugin-only fix and uses no private agent/request-surface API. The existing skipWhenMainModelSupportsImage option remains a generic prompt-assembly approximation.

Exact per-attempt Auto/Always/Disabled visibility needs a Harness core contract exposing the prepared route/capabilities and request-lifetime tool restriction/guard lifecycle.

Because the released llm/stream contract describes loop request content as read-only, a fully first-class handoff should also gain a core-owned, session-logged image-reference projection/admission hook. This PR hardens the compatibility shim already shipped in v0.6.4.

[Gu-ZT]

已审查并以 squash 合并（e497aa5）。感谢贡献！

评审确认的三处真实修复：

1. 原生支持图片的路由不再被无条件降级为文本引用 —— 按请求用原始 resolveModelInfo 探测真实模态，仅纯文本/未声明路由回退到 [image: …] 引用，这正是 v0.6.4 行为里最需要修的一点。
2. admission 包装不再虚增辅助视觉路由自身的能力，误选纯文本视觉模型时 askVision 的守卫现在会正确报错。
3. 重入守卫从全局布尔改为请求局部 WeakSet，并发流互不干扰；disposer 幂等且不会覆盖其他插件后装的包装，拆除语义干净。

本地复核： npm test 14/14 通过、typecheck 与 build 干净；PR CI 三平台全绿。与我这边 v0.6.4 引入的 skipWhenMainModelSupportsImage（提示词装配层）无冲突 —— 两者作用层级不同，组合语义一致。

两个非阻塞的后续建议：

• CI 工作流目前只跑 typecheck/build/pack，新增的 npm test 没有接入 CI，建议后续在 quality 作业里加一步，避免回归测试腐化。
• 能力解析失败现在会让含图请求以流错误失败（此前是无条件改写）。agent-loop 场景下解析在请求准备阶段已成功，实际无影响；如想更保守，可在解析异常时回退到安全改写。

关于 PR 描述里提到的核心契约（把准备后的路由/能力发布给提示词装配、请求生命周期的工具限制、session-logged 图片引用投影挂接点）：方向认同，欢迎到 dsh 主仓开 issue 讨论。