Add test preflight and parameterize PostgreSQL connection in Makefile

[seanwevans]

Motivation

• Make tests robust and environment-agnostic by allowing PGDATABASE, PGHOST, PGPORT, and PGUSER to be configured instead of hardcoding -d postgres.
• Ensure local and Docker test runs perform the same prerequisite checks so test failures are actionable and consistent.

Description

• Parameterized the test DB connection in makefile by adding PGDATABASE, PGHOST, PGPORT, and PGUSER defaults and a shared PSQL helper that respects those variables.
• Added a test-preflight target that checks for pg_prove on PATH, verifies PostgreSQL connectivity using the configured connection variables, and checks that required extensions (pgcrypto, pg_trgm, plpython3u) are installed, emitting actionable error messages on failure.
• Made the test target depend on test-preflight and invoke pg_prove with explicit -h, -p, -U, and -d flags so test execution uses the same connection settings.
• Updated README.md and CONTRIBUTING.md with a minimal known-good local vs Docker testing matrix and notes that make test includes the preflight; Docker Compose already runs make test so containerized runs now execute the same preflight checks.

Testing

• Attempted to run make -n test to validate Makefile behavior, but the dry-run/build was blocked by a missing PGXS include path in the execution environment (/usr/lib/postgresql/16/lib/pgxs/src/makefiles/pgxs.mk).
• Attempted to run make test-preflight in the containerized environment but the full make execution could not complete due to the same PGXS/environment limitation, so preflight could not be fully exercised here.

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Codex Task