fix(qoder): pair tool calls whose assistant turn has null content

[metal-huang]

中文

改动概要

• 修复 Qoder 模型的多轮工具调用从第二轮起全部失败：content: null 的 assistant 轮次被网关拒绝 #296。
• Qoder 网关只在前一条 assistant 消息的 content 为字符串时，才把它携带的 tool_calls 登记为可配对。OpenAI 客户端对纯工具调用轮次发送 "content": null，导致每次重放工具历史都被拒：400 InvalidParameter: messages with role "tool" must be a response to a preceeding message with "tool_calls"（实测矩阵见 issue）。
• BuildPlainChatBody 现在会把带 tool_calls 的消息上的 nil content 改写为 ""。调用 id、工具结果和其他一切内容形状原样透传；不带 tool_calls 的普通 assistant 消息保持原样。

测试计划

• go test ./internal/providers/qoder/ -run NullContent — 新增 3 个用例：null+tool_calls 改写为 ""、无 tool_calls 的 null 保持不动、非空 content 保持不动
• go vet 对涉及包无告警
• Added changelog/unreleased/qoder-null-content-tool-pairing.md
• No tokens, auth blobs, raw captures, or host details in the diff

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English

Summary

• Fixes Qoder 模型的多轮工具调用从第二轮起全部失败：content: null 的 assistant 轮次被网关拒绝 #296.
• The Qoder gateway only registers a tool-call pairing when the assistant turn's content is a string. OpenAI clients send "content": null for a pure tool-call turn, so every replayed tool history is rejected with 400 InvalidParameter: messages with role "tool" must be a response to a preceeding message with "tool_calls" (measured matrix in the linked issue).
• BuildPlainChatBody now rewrites a nil content to "" on messages that carry tool_calls. Call ids, tool results, and every other content shape pass through untouched; a plain assistant message with null content keeps its shape.

Test plan

• go test ./internal/providers/qoder/ -run NullContent — 3 new cases: null+tool_calls becomes "", null without tool_calls is kept, non-empty content is kept
• go vet clean on the touched packages
• Added changelog/unreleased/qoder-null-content-tool-pairing.md
• No tokens, auth blobs, raw captures, or host details in the diff

[caigee-cmd]

LGTM. 审查结论：

正确性 — 上游网关只在 assistant 消息 content 为字符串时登记 tool_calls 配对；OpenAI 客户端对纯工具调用轮次发送的正是 "content": null，导致配对未登记、后续 role:"tool" 消息被拒。把该轮次的 nil content 改写为 "" 是对症的修复。

范围 — normalizeMessageContent 只在 len(message.ToolCalls) > 0 && message.Content == nil 时改写，其他所有内容形状（非空字符串、空字符串、无 tool_calls 的 null）原样透传。我确认 BuildPlainChatBody 是 qoder 包唯一的 chat 请求构造路径，不存在第二条需要同步修改的分支。

验证 — 本地跑过：go build ./...、go test ./...（全量通过）、go vet ./...、release-notes.py validate（exit 0）、3 个新测试全通过。gofmt 仅 internal/store/migrations.go 一处，为 main 既有基线，与本 PR 无关。

合并。