OAK-12365: Delete stale index document when a node's last aggregated property is removed

[bhabegger]

Summary

FulltextIndexEditor.addOrUpdate() calls makeDocument() on every commit touching a node. When the node's indexing rule still matches but its last indexable content came from a relative/aggregated property that was just removed, makeDocument() returns null (nothing to index) and addOrUpdate() previously did nothing further — leaving the stale document in the index indefinitely.

A direct (non-relative) property removal on the same node was already handled correctly: FulltextDocumentMaker.removeProperties() forces dirty=true in that case, so a rebuilt document is written instead of null. The gap is specifically the aggregated-content case, where the root node's own propertiesModified list stays empty.

Fix: in addOrUpdate(), when makeDocument() returns null on an update to a previously-existing node, delete the stale document. Guarded by feature toggle FT_OAK-12365 (enabled by default), following this codebase's convention for bug-fix toggles.

Covers both the OAK-12244 type-tracking-enabled path and the legacy (toggle-disabled) path, since both funnel through addOrUpdate().

Related but distinct from OAK-12244 (mixin add/remove rule transitions): this is a content transition — the rule still matches throughout, but the document's content disappeared.

Test plan

• LuceneIndexEditor2Test#nodeLosesLastAggregatedPropertyTriggersDocumentDeletion — unit test verifying deleteDocument() is called (new, RED→GREEN)
• LuceneIndexEditor2Test#nodeLosesLastAggregatedPropertyTriggersDocumentDeletionWhenMixinToggleDisabled — same, with FT_OAK_12244 disabled to confirm the legacy path is also covered
• PropertyIndexCommonTest#nodeLosesLastAggregatedPropertyDisappearsFromFulltextIndex — integration test using a fulltext CONTAINS() query (a plain equality query can't observe this bug, since Oak's query engine revalidates simple WHERE conditions against the live tree)
• Full oak-search + oak-lucene suites: 1250 tests, 0 failures, 0 errors

[bhabegger]

[AI] Addressed feedback from an internal automated review:

• Registered FT_OAK-12365 as a FeatureToggle on the Whiteboard in both LuceneIndexProviderService and ElasticIndexProviderService — it was declared but never wired, so the kill switch had no effect at runtime.
• Added a unit test verifying FT_OAK_12365_DISABLE actually suppresses the deletion.
• Added a unit test verifying that removing one of several aggregated properties does not delete the document (only fully-empty content should).
• Reset FT_OAK_12365_DISABLE in @Before/@After alongside the existing FT_OAK_12244_DISABLE reset.
• Added a debug log line on the new deletion path, matching the existing trace log on the update path.

Full oak-search + oak-lucene (1252 tests) and oak-search-elastic (512 tests) suites pass.