* fix(caldav): don't prune the whole window when no objects could be parsed
The post-sync prune deletes local origin=="caldav" rows in the window whose UID
the server didn't just return. With an empty seen_uids it falls back to
`uid.isnot(None)` — a match-all delete. That's right when the calendar is
genuinely empty, but when the server returns objects and every one fails to
parse (malformed iCal / an icalendar error), seen_uids is empty only because
nothing could be read, so the match-all branch silently deletes every local
event in the 90-day-back/365-day-forward window.
Track whether any object failed to parse and gate the prune with a small pure
helper `_should_prune_window(seen_uids, parse_failed)`: prune when something was
read, or when the calendar is genuinely empty (no objects, no parse errors), but
never when objects came back unreadable.
Adds tests/test_caldav_prune_parse_failure.py for the three cases.
* fix(caldav): skip the prune on any parse failure, not just total
Review follow-up (#3454): _should_prune_window returned True whenever seen_uids
was non-empty, so a partial parse failure (say 48 of 50 objects parse) still
pruned the 2 unreadable-but-still-upstream events, because their UIDs were absent
from seen_uids. Any parse failure makes seen_uids an incomplete view of the
server, so pruning against it is unsafe whether the failure is total or partial.
Skip the prune on any parse failure (return not parse_failed); only prune on a
clean read (a genuinely empty window is still safe to prune). Tradeoff: one
permanently-unparseable event pauses deletion mirroring until it is fixed, which
is the safe direction (false-keep beats false-delete).
Replace the now-incorrect "partial failure still prunes" assertion with a
partial-failure regression: one object parses, one fails, so the prune is
skipped and the unparsed event's local copy is not deleted.
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Co-authored-by: Kenny Van de Maele <kenny@kvandemaele.be>
Mazen Tamer Salah