Suite shard planning recovers or fails on present-but-empty test targets

Suite-granularity shard plans no longer silently drop test targets when xcodebuild -enumerate-tests lists a target present-but-empty. XCTestEnumerator now reads xcodebuild’s errors array and surfaces top-level @Test functions as well as classes; ShardPlanService re-enumerates any present-but-empty target in isolation and either recovers it (if a clean re-enumeration produces suites), or fails the plan loudly with modulesFailedToEnumerate when it cannot, instead of silently missing tests.