fix(iterator): fallback to iterator consistency level if not provided in options

[marlon-costa-dc]

Summary

• When creating a QueryIterator or SearchIterator, if consistency_level is not explicitly provided in the options, it now falls back to the iterator's own consistency_level instead of hardcoding 0.

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[mergify]

@marlon-costa-dc Thanks for your contribution. Please submit with DCO, see the contributing guide https://github.com/milvus-io/milvus/blob/master/CONTRIBUTING.md#developer-certificate-of-origin-dco.

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@marlon-costa-dc Please associate the related issue to the body of your Pull Request. (eg. “issue: #187”)