feat(aggregate): expose final flush

[ArunPiduguDD]

Summary

Expose Aggregate::flush_final publicly so wrappers can drain open event-time buckets during shutdown without constructing and driving a temporary transform stream.

References

Related: event-time aggregation support

Vector configuration

N/A — this only exposes an existing aggregate transform method.

How did you test this PR?

• cargo fmt --all -- --check
• make test FEATURES="transforms-aggregate" SCOPE="aggregate" (48 passed)
• FEATURES="transforms-aggregate" make check-clippy reaches an unrelated pre-existing clippy::unused_async failure in src/test_util/http.rs

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

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