Fix PostgreSQL 18 build failure in ambulkdelete

[siddiqueirshad]

Summary

• Fixes the PostgreSQL 18 build break reported in Build failure with PostgreSQL 18 beta 1 #375: vacuum_delay_point() now requires a bool is_analyze argument (postgres commit e5b0b0c).
• Adds a version-gated compatibility shim in ldb_ambulkdelete() that expands to vacuum_delay_point(false) on PG 18+, matching the established approach used by pgvector for the same API change.
• Leaves the zero-argument call site unchanged for PostgreSQL 12-17, so existing supported majors are unaffected.

Root cause

PostgreSQL 18 changed vacuum_delay_point from void vacuum_delay_point(void) to void vacuum_delay_point(bool is_analyze). Lantern's index vacuum path in lantern_hnsw/src/hnsw/delete.c still used the zero-argument form, so builds against PG 18 headers fail with too few arguments to function call.

Test plan

• Confirmed open issue Build failure with PostgreSQL 18 beta 1 #375 has no assignees and no competing open PR
• Confirmed REL_18_STABLE declares vacuum_delay_point(bool is_analyze) and REL_17_STABLE still declares the zero-arg form
• Confirmed the shim matches the call-site pattern used by pgvector's PG 18 fix
• CI matrix (PG 12-17) should remain green (shim is #if PG_VERSION_NUM >= 180000)
• Build against PostgreSQL 18 headers should succeed where Build failure with PostgreSQL 18 beta 1 #375 previously failed

Fixes #375

[siddiqueirshad]

Hi @Ngalstyan4, when you get a chance, could you please take a look at this PR? Thanks!