Shard plan endpoint returns a valid suite plan instead of a 500 when suites are not enumerated

Shard plan requests with suite granularity no longer fail with a 500 when the client does not enumerate test suites. After the CLI stopped booting every test bundle to list suites, suite-granularity requests arrived with a present-but-nil test_suites value, which slipped past the old [] default and crashed enumeration downstream. The shard plan endpoint now derives suites from server-side history in that case, so it returns a valid shard plan instead of the previous “Failed to create shard plan due to an unknown server response of 500” error. The module-granularity path got the same nil-coalescing fix and is protected against the same crash.