fix(topology): serialize enrichment table registry loads

[pront]

Summary

Serialize access to the process-global ENRICHMENT_TABLES registry while topology builds or enrichment-table reloads move through load / finish_load.

This fixes an internal race for concurrent in-process topology builds/reloads. The normal daemon reload path already serializes reload signals through the topology controller, so this is primarily exposed by plain cargo test and test harnesses that build topologies concurrently.

Vector configuration

N/A

How did you test this PR?

• make fmt
• cargo test -p vector topology::test::reload --no-default-features --features sources-prometheus,sinks-prometheus,sources-internal_metrics,sources-splunk_hec,enrichment-tables-memory
• Applied this change to fix(enrichment tables): preserve memory enrichment table state on reload #25547 and verified its previously failing reload module now passes under normal parallel cargo test
• make check-clippy

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

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.

References

• Related: fix(enrichment tables): preserve memory enrichment table state on reload #25547

Notes

None.