fix(coderd/x/chatd): checkpoint buffered message_parts to avoid stale replay (#25145)

enterprise/coderd/x/chatd/chatd.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"math"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -853,8 +852,9 @@ func buildRelayURL(address string, chatID uuid.UUID) (string, error) {
 	u.Path = fmt.Sprintf("/api/experimental/chats/%s/stream", chatID)
 	q := u.Query()
 	// Relays only need live message_part events, not the full
-	// history; pass after_id=MaxInt64 so the peer skips its snapshot.
-	q.Set("after_id", strconv.FormatInt(math.MaxInt64, 10))
+	// history; pass the relay sentinel so the peer skips its
+	// durable DB snapshot and delivers in-flight parts only.
+	q.Set("after_id", strconv.FormatInt(osschatd.RelaySentinelAfterID, 10))
 	u.RawQuery = q.Encode()
 	return u.String(), nil
 }