fix(gateway): preserve system block cache control during rewrite

[black-06]

摘要

在 rewriteSystemForNonClaudeCode 时, 不合并为字符串, 而是将原始的 system content 以 block 为单位迁移 (包括 cache_control)

背景

问题

在使用 sub2api 作为 base_url 测试缓存命中时发现, 如果手动控制 system 块缓存, 多次请求缓存无法命中

最小复现请求是:

{
    "model": "claude-opus-4-7",
    "max_tokens": 64,
    "system": [
        { "type": "text", "text": "system prompt 1 ..." },
        { "type": "text", "text": "system prompt 2 ...", "cache_control": { "type": "ephemeral" } }
    ],
    "messages": [
        { "role": "user", "content": "reply with one short sentence." }
    ]
}

sub2api 会将 system 注入到 messages 中,改写影响了对应的 cache_control:

{
    "model": "claude-opus-4-7",
    "max_tokens": 64,
    "messages": [
        {
            "role": "user",
            "content": [
                { "text": "[System Instructions]\nsystem prompt 1 ...\n\nsystem prompt 2 ...", "type": "text" }
            ]
        },
        { "role": "assistant", "content": "Understood. I will follow these instructions." },
        { "role": "user", "content": "Request 1: reply with one short sentence." }
    ],
    "system": ...
}

方案

将 system text block 整个迁移为 user message content block, 保留 cache_control 属性

{
    "model": "claude-opus-4-7",
    "max_tokens": 64,
    "messages": [
        {
            "role": "user",
            "content": [
-                { "text": "[System Instructions]\nsystem prompt 1 ...\n\nsystem prompt 2 ...", "type": "text" }
+                { "text": "[System Instructions]", "type": "text" },
+                { "text": "system prompt 1 ...", "type": "text" },
+                { "text": "system prompt 2 ...", "type": "text", "cache_control": { "type": "ephemeral" } },
            ]
        },
        { "role": "assistant" "content": "Understood. I will follow these instructions." },
        { "role": "user", "content": "Request 1: reply with one short sentence." }
    ],
    "system": ...
}

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