Fix: time-range filter on VTODO with DTSTART/DUE and CREATED/COMPLETED

visit_time_ranges() reuses a single "original_duration" variable for two
unrelated purposes: the DTSTART->DUE span and the CREATED->COMPLETED span.
When a VTODO carries all four properties (a completed task, which most
clients write with CREATED and COMPLETED), the second assignment clobbers
the first, and the DTSTART/DUE branch of the rfc4791-9.9 table then
reconstructs DUE as DTSTART + (COMPLETED - CREATED).

The elif chain already implements the RFC table correctly (DTSTART/DUE
take precedence over CREATED/COMPLETED), so the CREATED/COMPLETED value
is never wanted there. Keep it in its own variable.

Effect: such a VTODO is filtered against a bogus interval, both in
calendar-query REPORT and in item.find_time_range() (the enclosing range
cached for the storage prefilter), so completed tasks go missing from -
or wrongly appear in - client results.

Co-Authored-By: Claude <noreply@anthropic.com>

radicale/tests/static/todo10.ics

@@ -0,0 +1,13 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Radicale//NONSGML Radicale Server//EN
+BEGIN:VTODO
+UID:todo10
+SUMMARY:todo10
+DTSTART:20130901T180000Z
+DUE:20130903T180000Z
+CREATED:20130901T170000Z
+COMPLETED:20130901T173000Z
+STATUS:COMPLETED
+END:VTODO
+END:VCALENDAR