fix(claude-code-proxy): 合并多条 system 消息为单条前置消息 / merge multiple system messages into one leading message

[WenhuaXia]

摘要 / Summary

Claude Code 2.1.x 把主 prompt 放在顶层 system 字段，同时会把额外的 role: 'system' 消息（如 ToolSearch 延迟工具提示）注入到 messages 中间。anthropicToOpenAiChat 原样保留该消息，产生 [system, user, system] 顺序，vLLM 等强制 system 在首位的 provider 返回 400：System message must be at the beginning。

Claude Code 2.1.x places its primary prompt in the top-level system field but also injects additional role: 'system' messages mid-conversation (e.g. the ToolSearch deferred-tools notice). anthropicToOpenAiChat kept that message in place, producing [system, user, system] order — rejected by vLLM-backed providers with 400 System message must be at the beginning.

修复 / Fix

新增 consolidateAnthropicChatSystemMessages()：合并所有 system 消息为单条前置消息（顶层 system 字段优先，其余按原顺序合并）。镜像 PR #2688 对 codex-proxy 的同类修复。

Adds consolidateAnthropicChatSystemMessages(), keeping exactly one leading system message (top-level field first, additional in-message systems merged in original order). Mirrors the earlier codex-proxy fix in PR #2688.

验证 / Verification

• 端到端复现：用 mock 抓包捕获真实 Claude Code 2.1.240 的 /v1/messages 请求，跑已部署的转换函数，确认产生 [system, user, system] 且 vLLM 返回 400
• 修复后同一 payload 转换出 [system, user]，vLLM 返回 200
• 新增 2 个回归测试（多 system 合并 + 单 system 保持原位）
• 43 个 adapter 测试全部通过
• End-to-end repro with a captured real Claude Code 2.1.240 payload: pre-fix 400, post-fix 200 from the vLLM endpoint
• 2 new regression tests; all 43 adapter tests pass

测试计划 / Test Plan

• npm run build（type-check 通过）
• vitest run tests/server/agent-runner-adapters.test.ts（43 passed）