Integrate PostgreSQL 19 features to Citus

[ihalatci]

Umbrella for PostgreSQL 19 feature integration in Citus. Build, compatibility, packaging, and CI stabilization are tracked separately in #8597 and are complete for Beta 4.

PostgreSQL 19 Beta 4 baseline

Beta 4 retained REPACK, JSON COPY, EXPLAIN IO, eager aggregation, constraint enforceability, and asynchronous I/O/read streams. It reverted FOR PORTION OF and partition MERGE/SPLIT.

Compatibility landed through #8870 and #8881. Post-merge validation is green:

• Build & Test: https://github.com/citusdata/citus/actions/runs/36406613062
• Nightly cassert: https://github.com/citusdata/citus/actions/runs/36406742370

Active Beta 4 integration work

Issue	PR	State	Remaining action
#8756	#8805	REPACK lock-order guardrail remains relevant	Rebase onto current main; refresh Beta 4 second-session lock regression; review and merge independently
#8758	#8808	Narrowed to retained ALTER CONSTRAINT enforceability guardrail	Rebase; remove unrelated DETACH changes; add focused Beta 4/cross-version coverage; review
#8770	#8807	EXPLAIN IO remains relevant	Restack directly onto current main; remove closed #8806 ancestry; refresh Beta 4 expected output; review

Completed

Issue	PR	Result
#8760	#8765	JSON COPY coordinator formatting/protocol fix merged and present on main
#8759 (table-grantor portion)	#8767	Explicit table GRANTED BY propagation merged and present on main; remaining #8766 work is version-neutral and tracked outside this umbrella

Deferred design

Issue	PR	State
#8772	—	Distributed REPACK CONCURRENTLY remains deferred pending a nontransactional placement execution model, per-node admission control, rebalancer exclusion, and explicit partial-failure/retry semantics

Removed or reclassified after Beta 4

Issue	PR	Disposition
#8761	#8763	Closed as obsolete because Beta 4 reverted FOR PORTION OF
#8771	#8806	Closed: eager aggregation remains upstream, but no active Citus defect was demonstrated and current Beta 4 CI is green
#8773	—	Closed/deferred: pg_stat_io already observes columnar reads and measured AIO benefit did not justify integration; the version-neutral buffer-cache pollution finding is tracked separately
#8759	#8766	Remaining shared-object REVOKE ordering work is version-neutral; removed from the pg19_features label and should land independently
#8762	—	Reproduces on PG17/18/19; removed from the pg19_features label and should be implemented/backported as a general deparser correctness fix

Completion rule

This umbrella is complete when the three active guardrail/adoption PRs are resolved and the deferred REPACK design has an explicit product disposition. Version-neutral bugs and future-major features do not block PostgreSQL 19 feature integration.