Preserve store flags on rollback

[Copilot]

Closes #7796.

Rollback was mutating store-level flags, contributing to unsafe cross-thread access around snapshot/chunk decisions. In 7.0 chunk boundaries are leader-decided, so rollback can leave these flags untouched.

• KV rollback

  • Stop clearing SNAPSHOT_AT_NEXT_SIGNATURE during Store::rollback().
  • Rollback now restores versioning state without changing pending store-level chunk/snapshot intent.

• Regression coverage

  • Extend Ledger entry chunk request coverage to assert rollback preserves the snapshot flag and still requests a chunk at the next signature.

store.rollback({store.commit_view(), store.current_version() - 1}, store.commit_view());

REQUIRE(store.flag_enabled(
  ccf::kv::AbstractStore::StoreFlag::SNAPSHOT_AT_NEXT_SIGNATURE));