Faster, more reliable suite-level test sharding

Tuist no longer boots every test bundle just to plan suite-level test shards. Instead, the server builds the shard plan from your project’s built .xctestrun and historical CI test data, which avoids slow, flaky enumeration under simulator contention. As a side benefit, the final shard now acts as a catch-all: previously unknown, renamed, or newly added suites are guaranteed to run instead of being silently dropped.