feat(component-failover-adoption): reconcile N_MAX_MASTERS with MAX_HA_CANDIDATES

N_MAX_MASTERS (client.h) now derives from config.h's MAX_HA_CANDIDATES
instead of an independently-maintained literal (3 vs 4 previously).
Fixes worker's and scheduler's hardcoded `> 3` startup checks to
reference the constant symbolically, and updates the client-retry
test fixture so it doesn't exercise an uninitialized 4th slot.

Task 1.1 of component-failover-adoption (Requirement 1 AC7).

src/include/client.h

@@ -6,6 +6,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "config.h"
 #include "netio.h"
 #include "request.h"
 #include "xce/types.h"
@@ -25,7 +26,9 @@ typedef void* xcecli_ctx_t;
  */
 typedef int (*xcecli_reconnect_cb)(xcecli_ctx_t ctx, void* arg);
 
-#define N_MAX_MASTERS (3)
+/* N_MAX_MASTERS derives from config.h's MAX_HA_CANDIDATES so the two limits
+ * can never silently diverge (Req 1 AC7, component-failover-adoption). */
+#define N_MAX_MASTERS MAX_HA_CANDIDATES
 
 typedef struct {
     int   nmasters;