Makefile: add PG_PROVE wrapper and check-pg_prove preflight for SQL tests

[seanwevans]

Motivation

• Ensure SQL test targets honor configured connection settings instead of hardcoding -d postgres.
• Make test invocation fail fast with a clear remediation when the pg_prove runner is not available.

Description

• Add a PG_PROVE wrapper that forwards PGHOST, PGPORT, PGUSER, and PGDATABASE into the pg_prove invocation.
• Add a check-pg_prove preflight target that verifies pg_prove is present and emits an actionable install hint when missing.
• Make test-core, test-integration, test-performance, test-one, and test-one-verbose depend on check-pg_prove and use $(PG_PROVE) (including --verbose for the verbose runner).

Testing

• Ran pytest -q which discovered no Python tests in this repository.
• Ran make test-core in this environment and it now fails early with the preflight message pg_prove not found. Install pgTAP test runner (e.g., apt install libtap-parser-sourcehandler-pgtap-perl). and exits with code 127, demonstrating the preflight check is working.
• No further automated SQL test runs were executed because the environment does not provide pg_prove or PGXS build files.

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Codex Task