feat(history): 会话列表lastMessage预览(B3) — 单查询无N+1

- history/session_preview.go: SessionMeta.LastMessage + 单条SQL(LEFT JOIN取每会话ORDER BY created_at DESC,rowid DESC LIMIT 1,走覆盖索引)
- ListSessions 迁出重写(签名不变), content折叠空白截前60rune
- gateway: historySessionSummary 增 lastMessage{role,content,createdAt}(omitempty), /sessions与/admin/sessions共用
- 新增测试(session_preview_test, history_lastmessage_test)
- Docs/api-client.md v0.22 同步

internal/runtime/history/session_preview_test.go

@@ -0,0 +1,134 @@
+package history
+
+import (
+	"context"
+	"strings"
+	"testing"
+)
+
+// sessionOf returns the SessionMeta of one session from ListSessions, failing the test
+// when the session is absent.
+func sessionOf(t *testing.T, st *Store, id string) SessionMeta {
+	t.Helper()
+	sessions, err := st.ListSessions(context.Background())
+	if err != nil {
+		t.Fatalf("list sessions: %v", err)
+	}
+	for _, m := range sessions {
+		if m.ID == id {
+			return m
+		}
+	}
+	t.Fatalf("session %q not found in %+v", id, sessions)
+	return SessionMeta{}
+}
+
+func TestListSessionsLastMessagePreview(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	ctx := context.Background()
+
+	// The empty main session has no preview.
+	if got := sessionOf(t, st, MainSessionID); got.LastMessage != nil {
+		t.Fatalf("empty main LastMessage = %+v, want nil", got.LastMessage)
+	}
+
+	sub, err := st.NewSubSession(ctx, "owner")
+	if err != nil {
+		t.Fatalf("new sub: %v", err)
+	}
+	if err := st.Append(ctx, Message{SessionID: MainSessionID, Role: RoleUser, Content: "first", CreatedAt: 1000}); err != nil {
+		t.Fatalf("append main user: %v", err)
+	}
+	if err := st.Append(ctx, Message{SessionID: MainSessionID, Role: RoleAssistant, Content: "latest reply", CreatedAt: 2000}); err != nil {
+		t.Fatalf("append main assistant: %v", err)
+	}
+	if err := st.Append(ctx, Message{SessionID: sub, Role: RoleUser, Content: "sub question", CreatedAt: 1500}); err != nil {
+		t.Fatalf("append sub: %v", err)
+	}
+
+	// Main stays first and carries its newest message, not the first one.
+	sessions, err := st.ListSessions(ctx)
+	if err != nil {
+		t.Fatalf("list sessions: %v", err)
+	}
+	if len(sessions) != 2 || sessions[0].ID != MainSessionID {
+		t.Fatalf("sessions = %+v, want main first", sessions)
+	}
+	main := sessions[0].LastMessage
+	if main == nil || main.Role != RoleAssistant || main.Content != "latest reply" || main.CreatedAt != 2000 {
+		t.Fatalf("main LastMessage = %+v, want assistant/latest reply/2000", main)
+	}
+	subPreview := sessions[1].LastMessage
+	if subPreview == nil || subPreview.Role != RoleUser || subPreview.Content != "sub question" || subPreview.CreatedAt != 1500 {
+		t.Fatalf("sub LastMessage = %+v, want user/sub question/1500", subPreview)
+	}
+
+	// A sub session created after the main one but never written has no preview.
+	empty, err := st.NewSubSession(ctx, "owner")
+	if err != nil {
+		t.Fatalf("new empty sub: %v", err)
+	}
+	if got := sessionOf(t, st, empty); got.LastMessage != nil {
+		t.Fatalf("empty sub LastMessage = %+v, want nil", got.LastMessage)
+	}
+}
+
+func TestListSessionsLastMessageTruncatesAndCollapses(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	ctx := context.Background()
+
+	long := strings.Repeat("字", lastMessagePreviewRunes+10) + "\n\n尾部"
+	if err := st.Append(ctx, Message{SessionID: MainSessionID, Role: RoleUser, Content: long, CreatedAt: 1000}); err != nil {
+		t.Fatalf("append long: %v", err)
+	}
+	want := strings.Repeat("字", lastMessagePreviewRunes) + "…"
+	if got := sessionOf(t, st, MainSessionID).LastMessage; got == nil || got.Content != want {
+		t.Fatalf("truncated preview = %+v, want %q", got, want)
+	}
+
+	// Whitespace runs (including newlines) collapse to single spaces, and a short
+	// message is returned whole.
+	sub, err := st.NewSubSession(ctx, "owner")
+	if err != nil {
+		t.Fatalf("new sub: %v", err)
+	}
+	if err := st.Append(ctx, Message{SessionID: sub, Role: RoleUser, Content: "  你好\n\t世界  ", CreatedAt: 1000}); err != nil {
+		t.Fatalf("append spaced: %v", err)
+	}
+	if got := sessionOf(t, st, sub).LastMessage; got == nil || got.Content != "你好 世界" {
+		t.Fatalf("collapsed preview = %+v, want %q", got, "你好 世界")
+	}
+}
+
+func TestListSessionsLastMessageTieBreaksByInsertion(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	ctx := context.Background()
+
+	// Two messages sharing a millisecond timestamp: the later insert is the newest.
+	for _, content := range []string{"older", "newer"} {
+		if err := st.Append(ctx, Message{SessionID: MainSessionID, Role: RoleAssistant, Content: content, CreatedAt: 5000}); err != nil {
+			t.Fatalf("append %s: %v", content, err)
+		}
+	}
+	if got := sessionOf(t, st, MainSessionID).LastMessage; got == nil || got.Content != "newer" {
+		t.Fatalf("tie-break preview = %+v, want newer", got)
+	}
+}
+
+func TestPreviewContentBounds(t *testing.T) {
+	t.Parallel()
+	if got := previewContent(""); got != "" {
+		t.Fatalf("empty preview = %q, want empty", got)
+	}
+	exact := strings.Repeat("a", lastMessagePreviewRunes)
+	if got := previewContent(exact); got != exact {
+		t.Fatalf("exact-length preview changed: %q", got)
+	}
+	over := strings.Repeat("a", lastMessagePreviewRunes+1)
+	if got := previewContent(over); got != exact+"…" {
+		t.Fatalf("over-length preview = %q, want ellipsis", got)
+	}
+}