PostgreSQL 18 introduces self-join elimination optimization that breaks
ParadeDB's context-sensitive score() functions by eliminating duplicate
table scans, causing score() to return the same value for both instances.

This implementation adds self-join detection and score() detection to the
existing planner hook (which handles window function replacement) and
uses a RAII guard to temporarily disable enable_self_join_elimination
when both conditions are met.

Changes:
- Add BoolGucGuard RAII pattern for safe GUC management
- Add has_self_join() to detect same table appearing multiple times with recursive subquery support
- Add has_score_function() to detect score() usage in queries
- Integrate PG18 self-join protection into existing paradedb_planner_hook
- Preserve all existing window function replacement functionality

The guard automatically restores the original GUC value when it goes out
of scope, ensuring configuration changes don't leak beyond their intended
scope.

This commit fixes two issues caused by rebase conflicts:

1. Restored pgvector upgrade to v0.8.1 for system Postgres installations
   - Line 127 in test-pg_search.yml was reverted to v0.8.0 during rebase
   - pgvector v0.8.1 is required for PostgreSQL 18 compatibility

2. Added missing pgrx CI configuration for PostgreSQL 18
   - Previously only had pgrx tests for PG 17
   - PG 18 tests were only running with system Postgres
   - Now includes pgrx-managed Postgres tests for PG 18

… hooks

PostgreSQL 18 introduced automatic rewriting of OR clauses to ScalarArrayOpExpr
(ANY) during match_clause_to_indexcol. This caused crashes in pg_search's planner
hooks which expected only scalar datums.

Changes:
- Enhanced from_datum_resilient() to detect and handle array datums
- Arrays are converted to Boolean OR queries: Boolean{should: [q1, q2, ...]}
- Added coerce_scalar_array() to safely detect array vs scalar datums
- Updated planner hooks (query_input_restrict, query_input_support) to use
  universal datum decoder
- Enhanced custom scan path with explicit scalar_array_use_or handling
- Maintains backward compatibility with PG14-17 scalar queries

This fix allows both user-written ANY() and planner-generated ANY() from
OR rewrites to work correctly on PostgreSQL 18.

PostgreSQL 18 added self-join elimination code that calls score_funcoids()
on every query through the planner hook, even before CREATE EXTENSION runs.
The old implementation used regprocedurein which panics when functions don't
exist yet.

This fix:
- Creates try_score_funcoids() using LookupFuncName with missing_ok=true
- Returns None gracefully when functions don't exist (before CREATE EXTENSION)
- Follows the ParadeDB pattern from utils.rs:lookup_pdb_function()
- Fixes test_logical_replication test failure on PG18

PG17 and below don't have this issue because the self-join elimination
check code is wrapped in #[cfg(feature = "pg18")].

Fixes #3375