Enumerated collectors that yield zero items log SUCCESS indistinguishable from healthy #1837

Spun out of #1833's triage. The reason both empty-grid defects (#1833, and the Query Store issue filed alongside this one) went unreported for so long is that they are invisible in Collection Health: the zero-item path returns SUCCESS with rows_collected = 0 and no message, so "collected nothing for weeks" reads as healthy.

The per-database (RunsPerDatabase) branch already has an "every item failed, so rethrow as an error" guard (RemoteCollectorService.DefinitionRunner.cs:187-192). The enumerated branch has no equivalent: an enumeration that returns zero items — because the login can't enter any database, because every probe failed into an empty CATCH, or because a filter excluded everything — early-returns 0 as SUCCESS (DefinitionRunner.cs:218-222).

Sketch: when an enumerating collector's enumeration yields zero items on a target where that is surprising (e.g., the server has user databases), log a distinguishable status or at least a message on the SUCCESS row, so Collection Health can band it. Needs a little design care to avoid crying wolf on legitimately-empty servers.

Applies to both Lite and Darling (shared runner shape in DarlingCollectorRunner).

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