fix(executor): upstream_empty 不冷却账号 / do not cool accounts down for upstream_empty

[BrianBoyCN]

中文

问题现象

#286 合入后的实测中发现：当上游空流率高企时（一次实验中 6 发有 4 发空流，约 40-60%），qoder 账号出现频繁冷却，账号池被饿死，请求在长时间排队后才成功。

根因

upstream_empty（#286 引入的转发前守卫）目前按普通 unavailable 错误分类，走 Classify 的 default 分支：

• 默认 15 秒冷却（ParseRetryAfterHint(body, retryAfter, 15*time.Second)）
• Pool.MarkClassified 对同类错误的重复失败还会指数退避（15s → 更长）

但空流只说明 Qoder 网关本次没有产出内容，账号本身完全健康——凭据、配额、限流状态都正常，同一账号下一次请求就可能成功。把"上游瞬时故障"按"账号故障"冷却，在空流率高时会把整个账号池冻结。

修复

internal/executor/error.go 的 ClassifyError 中对 Code == "upstream_empty" 特例处理：

• Failover = true —— 本次请求仍然换一个账号服务（保留 fix(qoder): 转发前拒绝上游空流 / reject empty upstream streams before relaying #286 的收益）
• Cooldown = 0 —— 账号立即回到轮询池，不被惩罚

其他 unavailable 错误（连接被重置等真实账号层故障）保持原有 15 秒冷却不变。Pool.MarkClassified 中 Cooldown=0 天然不设置 DownUntil，无需改池逻辑。

测试

新增 TestClassifyErrorUpstreamEmptyNoCooldown：

• upstream_empty → Failover=true、Cooldown=0、RetryAfter=0
• 对照组：普通 unavailable 错误保留默认冷却（确认只有特例豁免）

go test ./...、go test -race、go vet ./...、gofmt 全部通过。

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English

Problem

Live testing after #286: when the upstream empty-stream rate spiked (4/6 requests in one sample, ~40-60%), qoder accounts were cooled down repeatedly and the pool starved — requests queued behind long cooldowns even though the accounts themselves were healthy.

Root cause

upstream_empty (the pre-relay guard from #286) is classified as a generic unavailable error, which applies the default 15s cooldown plus exponential backoff on repeat failures. But a heartbeat-only upstream stream says nothing about the account's health: credentials, quota, and rate-limit state are all fine, and the same account may succeed on the very next request. Cooling accounts for a transient upstream fault freezes the whole pool when the empty-stream rate is high.

Fix

Special-case Code == "upstream_empty" in ClassifyError: keep Failover = true (another account serves the turn) but set Cooldown = 0 so the account stays in the rotation. Generic unavailable errors keep their 15s cooldown. Pool.MarkClassified already treats Cooldown=0 as no DownUntil, so no pool changes needed.

Tests

New TestClassifyErrorUpstreamEmptyNoCooldown covers both the exemption and a control case (generic unavailable keeps its cooldown). go test ./..., -race, go vet, gofmt all pass.