SharedDatabases: Initiate only a single connection per database

[nilmerg]

Since support for PHPUnit 10 was added, the data provider is being evaluated per class usage and only once per run, meaning that the setup and tear down routines for the schema are processed multiple times. This on its own isn't a problem, but test fixtures will add up during execution of multiple independent test cases and this wasn't intended when this trait was introduced.

This change introduces a specific singleton to maintain a global state across different test cases in order to have only a single connection per database. It also orchestrates schema handling better since it will be done per test case again now and is even adjustable to support shared state between different test cases. Each transaction can also be put into a database transaction which is rolled back automatically to ensure that no state, that is not part of the schema set-up, influences subsequent tests.

Also drops the following methods that were not marked internal:

• getEnvironmentVariable
• getConnectionConfig

[nilmerg]

Claude approves this:

I reviewed the diff between main and this branch (commit 684152a) against the stated goal: making SharedDatabases maintain a single connection per database across test classes, instead of a new one per class (which is what caused fixtures to accumulate under PHPUnit 10).

Design checks out. The key fix is moving $connections out of the trait (where PHP gives every consuming class its own copy of a trait's static properties) into SharedDatabases\State, a real singleton class shared process-wide. I traced the two shared-schema scenarios by hand:

• Two unrelated test classes (no SchemaGroup): maintainSchema() computes a distinct group hash per class name, so each gets its schema dropped/recreated on first use — matches old behavior, but now against one shared connection.
• Two classes with the same #[SchemaGroup('x')]: they hash to the same group id within a run, so the second class's setUpBeforeClass() sees __test_group.name already matching and skips the drop/recreate, correctly preserving shared fixtures.
• State::uniqueGroup() salts the hash with uniqid() per process, so leftover __test_group rows from a previous run (stale DB) never accidentally match and skip schema setup.
• TransactionIsolation: rollback-then-beginTransaction() runs in setUp(), so each test's changes get undone at the start of the next test (or at the start of the next class's maintainSchema(), which unconditionally drains any dangling transaction first). No test ever executes without a fresh transaction on
  top.

I also verified the methods being called that aren't defined directly on Connection (inTransaction(), exec()) are legitimate — Connection::__call() proxies unknown methods straight to the underlying PDO object, and both exist on PDO. ERRMODE_EXCEPTION is set in BaseAdapter, so the try/catch (Throwable)
around the __test_group existence check is well-founded (missing table reliably throws).

No functional bugs found. The dropped getEnvironmentVariable/getConnectionConfig methods are called out explicitly in the commit message as an intentional breaking removal of un-internal-marked API, which lines up with what the diff does.