feat(llm): 每个 LLM 配置支持自定义会话头

新增 session_header_key:非空时每次 LLM 请求带上该 HTTP 头,头值为当前
运行的 session id(chat 会话=conv-<id>,worker=exp<x>-worker-i<intent>
等)。用于某些按 session-id 头做提示缓存/粘性路由的网关。

实现:在已有的 quotaAwareTransport 里从请求 context 读取 session id
(transcript.WithSessionID 挂上的)并注入,无需改 norma;同一共享 provider
也能按会话发出不同头值。DB 加 session_header_key 列(带旧库迁移),前端
LLM 配置弹窗新增输入框。

agent/session_header_test.go

@@ -0,0 +1,66 @@
+package agent
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/Autumn-27/norma/transcript"
+)
+
+// fakeRT records the request it saw and returns a minimal 200 response.
+type fakeRT struct{ seen *http.Request }
+
+func (f *fakeRT) RoundTrip(req *http.Request) (*http.Response, error) {
+	f.seen = req
+	return &http.Response{
+		StatusCode: 200,
+		Header:     make(http.Header),
+		Body:       io.NopCloser(strings.NewReader("")),
+	}, nil
+}
+
+func newReq(ctx context.Context) *http.Request {
+	req, _ := http.NewRequestWithContext(ctx, "POST", "https://api.example.com/v1/messages", strings.NewReader("{}"))
+	return req
+}
+
+func TestSessionHeaderInjectedFromContext(t *testing.T) {
+	base := &fakeRT{}
+	rt := quotaAwareTransport{base: base, sessionHeaderKey: "x-session-id"}
+	ctx := transcript.WithSessionID(context.Background(), "conv-42")
+	if _, err := rt.RoundTrip(newReq(ctx)); err != nil {
+		t.Fatalf("RoundTrip: %v", err)
+	}
+	if got := base.seen.Header.Get("x-session-id"); got != "conv-42" {
+		t.Fatalf("x-session-id = %q, want conv-42", got)
+	}
+}
+
+func TestSessionHeaderSkippedWhenKeyEmpty(t *testing.T) {
+	base := &fakeRT{}
+	rt := quotaAwareTransport{base: base} // no key configured
+	ctx := transcript.WithSessionID(context.Background(), "conv-42")
+	if _, err := rt.RoundTrip(newReq(ctx)); err != nil {
+		t.Fatalf("RoundTrip: %v", err)
+	}
+	// The header name is whatever the user would have set; with no key, nothing
+	// session-related is added. Assert the common key stays absent.
+	if got := base.seen.Header.Get("x-session-id"); got != "" {
+		t.Fatalf("unexpected session header %q with empty key", got)
+	}
+}
+
+func TestSessionHeaderSkippedWhenNoSessionID(t *testing.T) {
+	base := &fakeRT{}
+	rt := quotaAwareTransport{base: base, sessionHeaderKey: "x-session-id"}
+	// Context carries no session id (transcript persistence off).
+	if _, err := rt.RoundTrip(newReq(context.Background())); err != nil {
+		t.Fatalf("RoundTrip: %v", err)
+	}
+	if got := base.seen.Header.Get("x-session-id"); got != "" {
+		t.Fatalf("x-session-id = %q, want empty when no session id on context", got)
+	}
+}