Test suite sharding now plans faster from history without booting every bundle

Suite-level test sharding no longer boots every test bundle to discover suites, which could be slow and fail with simulator bootstrap errors under load. Instead, the server builds shard plans from the built .xctestrun modules and historical suite timing data, falling back to the default branch when a module is new on the current branch. The final shard is a catch-all that runs anything not explicitly assigned, so newly added or renamed suites are never dropped. The server only sends this catch-all shape to compatible CLI versions.