Preserve sync quorum in primary PDB without master selector

[annielzy]

Problem

Clusters with synchronous_mode_strict: true need master + 1 in-sync replica to accept writes. Setting pdb_master_label_selector: false keeps minAvailable: 1, which allows up to N-1 simultaneous evictions on an N-node cluster, enough to drop below sync quorum and make the cluster read-only during rolling node drains.

Change

When pdb_master_label_selector is false and synchronous_mode_strict is true, the primary PDB now sets:

minAvailable: synchronous_node_count + 1

This preserves the write quorum of primary plus required synchronous replicas during voluntary disruptions such as rolling node drains.

Default behavior is unchanged when pdb_master_label_selector is enabled/unset or synchronous_mode_strict is false.