pg19: build cleanly against PostgreSQL 19beta1

[sfc-gh-dachristensen]

Summary

Builds on top of #363 (add-pg19-ci) — that PR adds the PG19 build/test infrastructure with continue-on-error; this PR makes the code actually compile against PG19.

PG19 dropped a number of transitive header includes and reshaped a few APIs. Most of the diff is just adding the explicit #include that PG18 was getting transitively.
The handful of real API changes are version-gated behind #if PG_VERSION_NUM >= 190000.

Header reorg

PG19 dropped these from common transitive include paths, so the relevant .c files now include them explicitly:

Header	Why
utils/tuplestore.h	was reached via nodes/execnodes.h
access/htup_details.h	was reached via executor/tuptable.h
utils/hsearch.h	HASHCTL, hash_create, hash_search
catalog/pg_type_d.h	TEXTOID, INTERVALOID, etc.
utils/wait_event.h	WAIT_EVENT_CLIENT_READ
utils/timestamp.h	DatumGetTimestampTz, TimestampTzGetDatum, IntervalPGetDatum
storage/lock.h	LockAcquire, LockAcquireResult
storage/fd.h	AllocateFile, FreeFile

API changes

Wrapped behind #if PG_VERSION_NUM >= 190000:

• LWLockNewTrancheId() now requires a name argument; the separate LWLockRegisterTranche() helper is gone.
• ShmemInitHash() collapsed init_size / max_size into a single nelems.
• SIG_IGN → PG_SIG_IGN (pqsigfunc-typed sentinel) when used with pqsignal().
• pg_plan_query() gained a trailing ExplainState * (pass NULL).
• CopyFormatOptions.binary / .csv_mode were replaced by an enum .format field. Wrapped behind CopyOptsIsBinary / CopyOptsIsCsvMode macros in pg_compat.h
  so callers keep the older names.
• CopyHeaderChoice enum was removed; header_line is now a plain int with COPY_HEADER_TRUE/FALSE/MATCH constants. Reintroduced the typedef locally so the
  exported API signature stays stable.
• numeric_int4_opt_error() renamed to numeric_int4_safe() and switched from a bool * error flag to an ErrorSaveContext * (soft-error API). Wrapped via
  an inline shim that preserves the older bool * signature.

Drive-by

pgduck_server is a frontend program; the unused #include "nodes/nodes.h" in numutils.h started failing because PG19 changed nodes.h to use Datum in extern
declarations. Removed the vestigial include.

Test plan

Built locally against PG19beta1 cleanly for:

• pg_extension_base
• pg_map
• pg_lake_engine
• pg_lake_copy
• pg_lake

pg_lake_iceberg / pg_lake_table were not exercised locally because the avro library was not built; the same include patterns applied here cover the breakages they
exhibited under PG19, but CI is the source of truth.

• Once ci: add PostgreSQL 19beta1 to build/test matrix, bump minor versions #363 lands, drop the PG19 continue-on-error overrides and confirm the matrix is green.