fix(test): resolve the PHP vendor tree against a declared floor

ebb835d541 raised phpunit/phpunit from ^11.0 to ^13.3. PHPUnit 13 requires
PHP >=8.4.1 and ubuntu-latest ships 8.3.6, so composer install could not
resolve and test-unit-php failed on every CI run:

    - phpunit/phpunit[13.3.0, ..., 13.3.2] require php >=8.4.1
      -> your php version (8.3.6) does not satisfy that requirement.

Nothing in this repository installs a particular PHP. ensure_php_toolchain
takes whatever the distro php package is, so the vendor tree was resolved
against an ambient interpreter that no file named. A dependency floor above
that interpreter is then only discovered by the resolver, on the machine that
happens to have the oldest PHP.

Declare the floor and resolve against it. config.platform.php binds composer
everywhere, including Dependabot; require states the same for readers; the
phpunit constraint follows it. Packagist floors: 11 -> >=8.2, 12 -> >=8.3,
13 -> >=8.4.1.

Verified in the dev container: the pinned resolution downgrades phpunit
13.3.2 -> 12.5.34 and the suite passes unchanged, 26 tests, 56 assertions.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

composer.json

@@ -3,10 +3,16 @@
   "description": "PHPUnit dependencies for the PHP shipped in roles/*/files/php/.",
   "type": "project",
   "license": "MIT",
+  "require": {
+    "php": ">=8.3"
+  },
   "require-dev": {
-    "phpunit/phpunit": "^13.3"
+    "phpunit/phpunit": "^12.0"
   },
   "config": {
+    "platform": {
+      "php": "8.3"
+    },
     "sort-packages": true
   },
   "scripts": {

utils/install/php.py

@@ -19,7 +19,7 @@ _PHPUNIT_EXTENSIONS = ("dom", "mbstring", "xmlwriter")
 def ensure_php_toolchain() -> None:
     """Install the PHP interpreter, Composer and the extensions PHPUnit needs.
 
-    ``phpunit/phpunit ^13`` declares ``_PHPUNIT_EXTENSIONS`` as platform
+    ``phpunit/phpunit`` declares ``_PHPUNIT_EXTENSIONS`` as platform
     requirements and Composer refuses to resolve the vendor tree without them,
     so they are part of the toolchain rather than of the suite consuming it.
     A distro ships them as packages separate from the interpreter, which is why