test: avoid reusable Neo4j index ids

[savasp-agent]

Summary

• remove the nondeterministic assumption that every dropped Neo4j schema index receives a new internal ID
• record and assert the exact DROP INDEX ... IF EXISTS command set for every configured managed index
• retain coverage that rebuilt indexes are online and usable and that external indexes and constraints are preserved

Why

The final #393 certification run observed Neo4j 5.26.28 reuse one internal schema index ID after recreation. The same strict suite passed immediately beforehand, and the focused test passed six consecutive isolated reruns afterward. Internal schema IDs are not a stable behavioral oracle for replacement.

The replacement assertion observes the schema commands issued by RecreateManagedIndexesAsync, proving that every configured managed index is dropped before its stable postconditions are checked. This remains test-only: no provider behavior or public contract changes.

Validation

• focused managed-index recreation test: 1 passed
• strict Neo4j lane: 616 passed, 0 failed, 0 skipped
• pre-push gate: Release warnings-as-errors build and format verification passed

Closes #532.