fix(phpunit): scope file-only testsuites to declared files

[chubes4]

Problem

wp_codebox_phpunit_parse_config() (generated by phpunitConfigDiscoveryPhp() in packages/runtime-playground/src/phpunit-command-handlers.ts) defaults $directories to the whole test root and only overwrites it when a <testsuite> declares <directory> elements. A <testsuite> that declares only <file> entries never triggers that overwrite, so $directories silently stays at the whole-test-root default and discovery unions the declared files with a recursive scan of the entire tree.

Real PHPUnit does not do this: a <testsuite> with only <file> entries and no <directory> runs exactly those files and performs no directory-based discovery at all. That's the reference behaviour this PR restores.

The three cases

Config declares	Before this PR	After this PR
<directory> (with or without <file>)	uses declared directories	unchanged
only <file>, no <directory>	falls back to whole-test-root scan, unioned with declared files	fixed — runs only the declared files, no directory scan
neither	whole-test-root default	unchanged

Evidence

On Extra-Chill/extrachill-events, booking-alpha-concurrency declares exactly one file (InternalBookingHoldConcurrencyMySQLProof.php, 2 test methods) and no <directory>. A run of that suite reported 88 tests, 615 assertions — 44x the declared content. Three sibling suites (booking-mysql, booking-concurrency, booking-alpha) have the same shape; a fourth (managed-wordpress), which does declare a <directory>, scoped correctly and served as the control case that pointed at this discovery function specifically.

Downstream, the wide scan swept in a stub file three directories away from anything the failing suite's config named, causing Cannot redeclare function wp_get_ability() and fataling the entire suite before any of its actually-declared tests ran. That's tracked separately as Extra-Chill/extrachill-events#846 — this PR fixes the root cause it surfaced; the stub itself is fixed independently over there.

The fix

Two lines in phpunitConfigDiscoveryPhp():

if (!empty($config_dirs)) {
    $directories = $config_dirs;
    ...
} elseif (!empty($files)) {
    $directories = array();
}

$config_dirs and $files are already accumulated separately from <directory> and <file> elements across all matched testsuites. This only adds the missing branch: when no <directory> was declared but at least one <file> was, clear $directories instead of leaving it at the whole-tree default. Configs declaring <directory> (case 1) and configs declaring neither (case 3) take the same branches as before and are unaffected.

This is the shared discovery-config parser used by both the plugin PHPUnit path (wp_codebox_phpunit_parse_config) and the core PHPUnit path (core_pg_parse_phpunit_config) — both call sites get the fix since they share this generator function.

Tests

Added to tests/phpunit-discovery-only.test.ts (extended, following its existing execFileSync("php", ...) pattern already used in tests/phpunit-project-autoload.test.ts to execute the generated PHP directly rather than just type-checking the TS template):

• case: only <file> declared runs exactly that file, no directory scan (the fix) — the regression test. Builds a tree with an unrelated *Test.php file sitting in the test root next to a file-only suite's declared file, and asserts discovery returns only the declared file. Without the fix in this PR, this fails with the unrelated file swept in (verified locally by reverting the fix and re-running).
• case: <directory> declared stays scoped to it (unchanged) — control case, proves directory-declaring suites are unaffected.
• case: neither <directory> nor <file> keeps the whole-test-root default (unchanged) — control case, proves the whole-tree default (used by configless/implicit discovery) is preserved.

This exercises the default discovery path (no --testsuite selection), which is the path the production bug actually lived in. It's distinct from the existing selected_testsuites coverage in tests/phpunit-project-autoload.test.ts, where $directories is unconditionally reset to array() before config parsing and was never affected by this bug — that's why that existing test passed both before and after this fix.

Ran the full existing PHPUnit-related test files (phpunit-discovery-only, phpunit-project-autoload, phpunit-result-paths, phpunit-runtime-failure-diagnostics, phpunit-runtime-rejection, phpunit-structured-evidence) plus npm run build — all pass.

Related

• Fixes PHPUnit discovery scans the whole test root when a testsuite declares only <file> entries, silently sweeping in unrelated tests #2516
• Downstream fatal this caused: test: sandbox phpunit suites are red on every current PR — three distinct failure modes before PHPUnit executes Extra-Chill/extrachill-events#846