fix(messages): omit empty thinking signature instead of signature: "" (strict clients reject it)

[warelik]

中文 TL;DR

上游从不产出 Anthropic thinking signature，代理原来发 signature:""，严格客户端
（如 Grok Build 的 messages 后端）会判为非法。改为无真实签名时省略该字段；上游给了
真实签名则原样转发。

Summary

signature: "" on an Anthropic thinking block is a proxy-synthesized placeholder,
not a real Anthropic signature (our upstream never emits one). Some strict clients
reject the empty string as invalid. Omit the field when there's no real signature;
forward a real one verbatim when upstream provides it (forward-compat).

Fix

• src/handlers/messages.js:
  • non-stream openAIToAnthropic: build the thinking block without signature
    unless reasoning_signature is present.
  • AnthropicStreamTranslator: emit signature_delta only when a real
    delta.reasoning_signature arrived (drop the empty-string fallback).

Test plan

• test/messages.test.js: signature/delta absent without a real signature
  (non-stream + streamed); real-signature round-trip preserved
• test/anthropic-input-validation.test.js: no regressions
• Full suite: npm test → 2761 pass / 0 fail

Branch is on current master (v3.5.0), applies cleanly.

[dwgx]

已合并,谢谢 🙏 这条改得对,而且原来的注释其实自己就埋了线索 —— 它写着空串是 "Foxfishc-verified safe fallback",但那个验证只覆盖了当时那个客户端。signature: "" 在 Anthropic spec 里本来就不是一个合法值,严格客户端按 spec 校验就会拒,所以"缺失时省略字段"比"填一个占位符"更符合协议本意。

流式那半修得尤其对:原来是无条件发 signature_delta,现在改成只在真有 reasoning_signature 时才发。测试也跟着从"必有一个空串 signature_delta"改成"无真实签名时不发",同时保留了 thinking_delta 与 content_block_stop 的顺序断言 —— 该松的松、该守的守,没有顺手把顺序保证一起删掉,这个分寸是对的。

上游将来真给 signature 仍原样透传,前向兼容没丢。

已加入致谢名单,评级 B+。