feat: add JetStream cluster stream replication and placement tests (Go parity)
Adds 97 tests across two new files covering stream replication semantics (R1/R3 creation, replica group size, publish preservation, state accuracy, purge, update, delete, max limits, subjects, wildcards, storage type) and placement semantics (replica caps at cluster size, various cluster sizes, concurrent creation, stepdown resilience, long names, re-create after delete).
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