test(496,484): record projection failures during enumeration, not eagerly [decisions-edit]

Re-review Low. The substitute incremented the failure counter inside .Returns(...), i.e. when
the enumerable was handed out, while the real paginator records from ProjectToMusicVideo's catch
DURING enumeration. A refactor that snapshotted Count before the enumeration completed would
then break production while both replacement tests kept passing — exactly the regression the
guard exists to prevent.

Moves the recording into an async iterator, and corrects the decision record to describe #484's
removed music-video test accurately and name its two replacements.

docs/decisions.md

@@ -3905,7 +3905,13 @@ owner's sweep **destroyed** the row the other library still served. This is the
   the protection — which is what this scanner did until the counter was threaded through
   (`MediaServerProjectionFailureCounter`, one per enumeration, never a field on the singleton api client). The
   single guard call covers the legacy path diff too, which is if anything more exposed: a legacy row has no etag
-  to fall back on.
+  to fall back on. #484's own music-video test asserted the pre-#496 hard delete
+  (`FindMusicVideoPaths`/`DeleteByPath`) and could not survive this change; it is replaced by
+  `ScanLibrary_Should_Not_Sweep_When_The_Projection_Reported_Failures` (identity sweep) and
+  `..._Should_Not_Sweep_Legacy_Rows_When_The_Projection_Reported_Failures` (legacy sweep). Their substitute
+  records failures **during** enumeration, not when the enumerable is handed out — an eager substitute would keep
+  passing if a refactor snapshotted `Count` before the enumeration completed, which is precisely the regression
+  the guard exists to prevent.
 - **Scope honesty: this is parity, not a total fix.** One file path is still one `MediaItem` row globally
   (`MediaFileAlreadyExists` is a global path-hash guard), so a second library serving the same file still gets
   no row of its own — exactly as for movies/TV. What changes is that the first library's sweep now *flags*