Fix silently-swallowed CI failures + resolve pre-existing PHPStan errors

[miguelcolmenares]

Summary

Two related fixes found while investigating why 3 real PHPStan errors in community-listings never failed CI:

1. The 3 PHPStan errors themselves, resolved via real refactors (no @phpstan-ignore suppressions):

   • SettingsPage::render_settings_page(): removed a dead-code truthy check — wp_count_posts() always returns an object per its own stub, never false.
   • CptRegistrar::register_graphql_meta_fields(): added the missing function_exists('register_graphql_field') guard (only register_graphql_object_type was checked, even though both are called — WPGraphQL always registers both together in practice, so this was never a runtime risk, but PHPStan can't know that without its own explicit check).
   • Same method: refactored the PHP-type → GraphQL-scalar mapping from an array + ?? fallback to a match expression. The array fallback was provably unreachable (every current META_FIELDS value is a mapped key) but had real defensive value for a future entry using an unmapped type — a match default arm preserves that behavior without PHPStan being able to prove it dead.

2. The Makefile bug that hid them. phpcs/phpstan/install/install-dev/build all looped for plugin in $(PLUGINS) without checking each iteration's exit status — only the last plugin's result reached Make. Since graphql-shortcode-support (last in $(PLUGINS)) always passed, community-listings' real PHPStan failures were silently swallowed. Confirmed via a CI log from before this fix: the PHPStan step printed Found 3 errors and returned with error code 1 for community-listings, then kept going and the job still reported success. Same root-cause class as the run-quality-checks.sh errexit bug found during the wp-coding-standards portfolio rollout.

   While fixing this, also found make phpcs (via composer run phpcs) disagreed with CI's own PHPCS step (which passes --warning-severity=0) — once exit codes stopped being swallowed, make phpcs started failing on pre-existing warnings CI is configured to ignore. make phpcs now calls vendor/bin/phpcs --warning-severity=0 directly, matching CI exactly.

Verified

• make test (install-dev + phpcs + phpstan) and make build both exit 0 across all three sub-plugins
• PHPCS: 0 errors, 0 warnings-that-CI-cares-about in all 3 sub-plugins
• PHPStan: 0 errors in all 3 sub-plugins (previously 3 in community-listings)
• A pre-PR core-review pass (read-only subagent) confirmed no dangling references to the removed $type_map variable, semantic equivalence of the match refactor against every META_FIELDS value, and valid Make/shell semantics for the exit-status propagation — it's what caught install/install-dev/build sharing the same bug, since my first pass only touched phpcs/phpstan

CHANGELOG

Also backfills [Unreleased] entries for #2 (Provider Listings meta box) and #3 (coding-standards adoption), which hadn't been documented yet, alongside this fix.