fix(collection): wait for flush completion instead of returning immediately

[marlon-costa-dc]

Summary

• The flush method previously returned immediately after sending the request.
• This PR updates it to poll GetFlushState and wait for the flush to actually complete (up to 30s timeout).
• This prevents race conditions where subsequent operations fail because the flush hasn't finished.

[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”)