fix(build): add missing fmt import to a2a_proxy.go

Build broken on main since d86b8fe — a2a_proxy.go uses fmt.Errorf()
(8 call sites) but the import was dropped during an isSafeURL refactor
merge. CI fails with "undefined: fmt" at lines 743-775.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

workspace-server/internal/handlers/a2a_proxy.go

@@ -6,6 +6,7 @@ import (
 	"database/sql"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"log"
 	"net"