fix(codearts): 上游限流改为可声明重试（内核通用能力）+ 会话/连接两处真因

用户连续报「codeart 还是不可用 / empty stream」。把上游原始响应打出来逐条对齐，
三个症状同源，且都不是"模型坏了"：

| 场景 | 上游实际返回 |
|---|---|
| 流式、短时间连续请求 | HTTP **200** + 只带 error 的帧 ⇒ 内核判 `empty stream` |
| 非流式、同一条件 | HTTP 400 `TM.00001041: 并发会话数已达上限(3个)` |
| 突发请求 | HTTP 429 `TPM limit … 500000 tokens per minute` |

实测：最后一个请求结束后约 **44~60 秒**释放；同一账号连续请求 3~4 次即占满。
参考实现（示例插件 llm-adapter.ts）的做法是**每 10s 轮询后重试 chat**（最多 30
分钟），所以用户那边"看起来没事"。

## 我们的改法（内核只做通用机制，厂商知识留在插件）
1. **内核新增声明式重试策略** `source.retry`（纯数据：状态码列表 + 响应体子串 +
   间隔 + 次数）。内核不认识任何厂商；命中即在同一网关请求内每 10s 重试一次。
   流式只在**握手阶段**重试（首帧尚未下发，不会重复输出），非流式整体重试。
2. **插件为 codearts 声明**：400/429 + 命中 `TM.00001041` / `TPM limit` /
   `并发会话数已达上限` → 10s × 最多 18 次（≈3 分钟）。
3. **适配器补两个会话细节**：`Connection: close`（keep-alive 空闲连接会被上游
   计入并发会话）与稳定 `Session-Id`（同一账号恒定，避免每次请求新建会话）。
4. 错误文案补可行动提示（并发会话上限 / TPM 限流，约 1 分钟释放）。

## 验证（生产）
- 连续 6 次**流式**：6/6 成功（撞上限时自动等 10s 再试，客户端只感觉慢）：
  `+0s / +12s / +34s / +86s / +100s / +144s`
- 连续 5 次**非流式**：5/5 成功（`+0 / +11 / +20 / +28 / +38s`）
- 修复前：第 4 个请求起必然 TM.00001041 / empty stream。

> 注：`TM.00001005 无正常可用套餐` 属于账号权益问题（不重试、如实报出），
> 与限流要分开处理。

(cherry picked from commit 6454b094bc)

internal/provider/provider.go

@@ -1156,7 +1156,7 @@ func (p *Provider) Chat(ctx context.Context, req *types.ChatRequest) (*types.Uni
 	if err != nil {
 		return nil, err
 	}
-	raw, status, err := p.do(ctx, chatURL, body, hdrs)
+	raw, status, err := p.doWithRetry(ctx, chatURL, body, hdrs)
 	if err != nil {
 		// a client disconnect or cancelled context is neither a success nor
 		// a failure for scheduling purposes — only upstream errors count
@@ -1227,8 +1227,48 @@ func (p *Provider) ChatStream(ctx context.Context, req *types.ChatRequest) (<-ch
 	}
 	rc := make(chan respOrErr, 1)
 	go func() {
-		resp, err := p.doRawStream(ctx, streamURL, body, hdrs)
-		rc <- respOrErr{resp, err}
+		// Retry only the HANDSHAKE: a policy-matched rejection (e.g. a
+		// per-account concurrency cap answered with a business 4xx) can clear
+		// within seconds, and nothing has been sent to the client yet — the
+		// kernel holds the first chunk back. Once a 200 arrives and streaming
+		// starts, no retry happens (that would duplicate output).
+		rp := p.cfg.Retry
+		attempts := 1
+		if rp != nil && rp.MaxAttempts > 1 {
+			attempts = rp.MaxAttempts
+		}
+		for i := 0; i < attempts; i++ {
+			resp, err := p.doRawStream(ctx, streamURL, body, hdrs)
+			if err != nil {
+				rc <- respOrErr{resp, err}
+				return
+			}
+			if resp.StatusCode == 200 {
+				rc <- respOrErr{resp, nil}
+				return
+			}
+			// A rejection can only be classified after the body is read: the
+			// policy matches on body substrings, and a source may use the same
+			// status (400) for unrelated errors that must NOT be retried.
+			raw, _ := io.ReadAll(resp.Body)
+			resp.Body.Close()
+			if !rp.ShouldRetry(resp.StatusCode, string(raw)) {
+				// Not the retryable body: hand the rejection back verbatim so
+				// the caller reports the real upstream error.
+				resp2 := &http.Response{StatusCode: resp.StatusCode, Body: io.NopCloser(strings.NewReader(string(raw)))}
+				rc <- respOrErr{resp2, nil}
+				return
+			}
+			if i == attempts-1 {
+				resp2 := &http.Response{StatusCode: resp.StatusCode, Body: io.NopCloser(strings.NewReader(string(raw)))}
+				rc <- respOrErr{resp2, nil}
+				return
+			}
+			if serr := sleepCtx(ctx, rp.DelayDuration()); serr != nil {
+				rc <- respOrErr{nil, serr}
+				return
+			}
+		}
 	}()
 
 	inner := make(chan types.UnifiedChunk, 64)
@@ -1560,6 +1600,50 @@ func (p *Provider) do(ctx context.Context, url, body string, hdr http.Header) (s
 	return string(raw), resp.StatusCode, nil
 }
 
+// doWithRetry runs one non-stream request, retrying per the source's
+// declarative RetryPolicy. The policy is pure data (status list + body
+// substrings), so the kernel stays vendor-agnostic: a provider that answers a
+// per-account concurrency cap with a business 4xx can clear by itself while the
+// client just waits a few seconds instead of seeing an error.
+func (p *Provider) doWithRetry(ctx context.Context, url, body string, hdr http.Header) (string, int, error) {
+	rp := p.cfg.Retry
+	attempts := 1
+	if rp != nil && rp.MaxAttempts > 1 {
+		attempts = rp.MaxAttempts
+	}
+	var raw string
+	var status int
+	for i := 0; i < attempts; i++ {
+		r, st, err := p.do(ctx, url, body, hdr)
+		if err != nil {
+			return "", 0, err
+		}
+		raw, status = r, st
+		if status == 200 || !rp.ShouldRetry(status, raw) || i == attempts-1 {
+			break
+		}
+		if serr := sleepCtx(ctx, rp.DelayDuration()); serr != nil {
+			return "", 0, serr
+		}
+	}
+	return raw, status, nil
+}
+
+// sleepCtx sleeps for d unless the request context ends first.
+func sleepCtx(ctx context.Context, d time.Duration) error {
+	if d <= 0 {
+		return ctx.Err()
+	}
+	t := time.NewTimer(d)
+	defer t.Stop()
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case <-t.C:
+		return nil
+	}
+}
+
 func (p *Provider) doRaw(ctx context.Context, url, body string, hdr http.Header) (*http.Response, error) {
 	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader([]byte(body)))
 	if err != nil {