PG19: port ruleutils into Citus as ruleutils_19.c

[ihalatci]

DESCRIPTION: Port upstream PG19 ruleutils.c into Citus deparser tree.

Mirrors #8010 (PG18) and #7725 (PG17). Replaces the ruleutils_19.c
placeholder from the build-foundation PR with a proper port of upstream
PG19's src/backend/utils/adt/ruleutils.c, produced by a 3-way merge
(git merge-file: ruleutils_18.c as base, upstream PG18 and upstream
PG19 as the two sides). All merge conflicts fell in blocks Citus had
already stripped from _18.c or in shard-aware deparse variants, and were
resolved by keeping the Citus side. The file keeps Citus' existing
curation (deparse hooks, shard-aware UPDATE/DELETE branches) while
picking up real upstream PG18->PG19 deparse changes.

Without this, Citus on a PG19 backend would deparse worker-bound SQL
using PG18 semantics -- silent corruption the moment a PG19-only node,
clause, or formatting decision appears in the tree.

The trailing #endif guard text is corrected to
"(PG_VERSION_NUM >= PG_VERSION_19) && (PG_VERSION_NUM < PG_VERSION_20)"
(the _17.c/_18.c snapshots carry stale guard comments; not propagated).
The generate_function_name fgc_flags fix is absorbed naturally by the
merge -- upstream PG19 already passes &fgc_flags at that call site.

Excluded from citus-style via .gitattributes (added in the
build-foundation PR): the file exceeds the CI uncrustify 10,000-line
limit and is upstream-derived.

Refs: #8597

---

Stacked on #8601 (pg19-build-foundation).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.76%. Comparing base (ab238fc) to head (23f44f2).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           pg19-build-foundation    #8602      +/-   ##
=========================================================
- Coverage                  88.76%   88.76%   -0.01%     
=========================================================
  Files                        288      288              
  Lines                      64315    64316       +1     
  Branches                    8091     8091              
=========================================================
- Hits                       57091    57087       -4     
- Misses                      4891     4894       +3     
- Partials                    2333     2335       +2     

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