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.

[sre-ci-robot]

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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.

[mergify]

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

[liliu-z]

When options.consistency_level is None and the collection default is Customized, this PR changes the wire value from 0 (Strong) to 4 (Customized) while use_default_consistency remains true. The iterator never consumes options.guarantee_timestamp (dead field at L72, L197), so the request carries Customized without the user-supplied timestamp that the proto spec requires (proto L1041). Whether this causes issues depends on server-side handling of use_default_consistency=true — which cannot be verified from this repo. The fix direction is correct for the 4 common levels (Strong/Session/Bounded/Eventually). Same pattern at L715, L961, L1403. Consider a follow-up to wire options.guarantee_timestamp into the request, or defensively reject Customized when the user hasn't set a consistency level.

[Copilot]

Pull request overview

This PR updates the iterator request construction so that QueryIterator and SearchIterator use the iterator’s configured collection consistency level when options.consistency_level is not explicitly provided, rather than defaulting the request field to 0.

Changes:

• Use self.consistency_level as the fallback value for consistency_level in iterator query/search requests when the option is unset.
• Make checkpoint file opening explicitly non-truncating and apply small idiomatic cleanups (map_err(Error::Io), simplifying an iterator sum, and an else if refactor).

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