feat: let a shard unit bundle several test classes

[maks-oleksyuk]

--shard splits test classes and weighs each one on its own. That holds while a test class is the whole unit of work, but not when the work a shard has to do spans several of them: splitting those classes across shards duplicates the work and leaves every shard with a partial result.

Mutation testing is the case that needs this. A mutation belongs to a source file, several test classes can cover that file, so the file is the unit and its covering test classes have to travel together. Related to #1691.

What changed

Shard::useTimings() accepts a richer value. Instead of a duration per test class, a unit may be ['time' => float, 'tests' => list<string>] under any key. A bare float still means a test class bundling only itself, so an existing shards.json and the behaviour of a plain --update-shards run are unchanged.

LPT partitioning now runs over units, and the --filter a shard receives is the union of the test classes its units bundle. Shard::selectedUnits() exposes the keys allocated to the current shard, so a plugin can narrow the work it generates to match.

Shard::useTimings() also takes optional metadata, stored next to the units and read back through Shard::metadata(). That lets a plugin carry a reference value from the unsharded run that produces the file into the sharded runs that consume it.

Three smaller fixes ride along. The timings file is written when a plugin supplied the units and the run exits non-zero, so a job whose only purpose is to record timings is not lost to an unrelated failure. The count reported by --update-shards now matches the number of entries actually written. The --list-tests subprocess no longer inherits --coverage arguments.

Testing

tests/Unit/Plugins/Shard.php covers unit normalisation, partitioning that keeps a bundle intact, both file shapes on read and write, precedence of plugin-supplied units, and the zero entries written for test classes that produced no work of their own.

Follow-ups

The plugin side that consumes this API lands in pestphp/pest-plugin-mutate#40, with the timeout it depends on in pestphp/pest-plugin-mutate#41. Together with #40 this closes the request in #1691.