Fix 500 when creating a suite-granularity shard plan with no enumerated suites

When you request a suite-granularity test shard plan without enumerating the test suites on the CLI side, the server used to respond with a 500 and the message “Failed to create shard plan due to an unknown server response of 500.” The server now falls back to deriving the suites from its own history in that case and returns a valid shard plan, so selective testing flows that omit the enumerated test_suites payload no longer fail.