feat(events): support RRULE recurrence in create/update/list

[realitix]

Summary

Adds RRULE recurrence support to createEvent, updateEvent, and
listEvents.

Changes

• createEvent accepts an optional recurrence parameter taking an
  iCal RRULE string (e.g. FREQ=WEEKLY;BYDAY=MO,TU,TH,FR or
  RRULE:FREQ=DAILY;COUNT=10). The RRULE: prefix is optional and added
  automatically if missing. Invalid rules are rejected with a clear error.
• updateEvent accepts the same recurrence parameter. Empty string
  or null clears the existing recurrence and turns the event into a
  one-shot; a non-empty string replaces the rule.
• listEvents / formatEvent now exposes the RRULE in clear text
  (without the RRULE: prefix) under a recurrence field on the master
  event, alongside the existing isRecurring boolean.

Implementation notes

• cal.createRecurrenceInfo() and cal.createRecurrenceRule() are not
  exposed on the cal namespace in Thunderbird's calUtils. The PR uses
  the XPCOM contracts (@mozilla.org/calendar/recurrence-info;1 and
  @mozilla.org/calendar/recurrence-rule;1) like the rest of the codebase
  already does for other calIxxx instances.
• Two small helpers were extracted to keep createEvent / updateEvent /
  formatEvent readable:
  • setRecurrenceOnItem(item, recurrence)
  • extractRRuleFromItem(item)

Test plan

• createEvent with recurrence: "FREQ=WEEKLY;BYDAY=MO": event appears
  every Monday in Thunderbird's calendar UI.
• createEvent with recurrence: "RRULE:FREQ=DAILY;COUNT=5": 5
  occurrences only.
• createEvent with recurrence: "GARBAGE": returns
  { error: "Invalid recurrence rule: ..." }.
• updateEvent with recurrence: "": existing recurring event becomes
  a one-shot.
• listEvents on a recurring master returns
  recurrence: "FREQ=WEEKLY;BYDAY=MO" (no RRULE: prefix) on the
  expanded occurrences.
• Non-recurring events return recurrence: null.

[TKasperczyk]

Thanks @realitix. Few items before merge:

• After feab2c7 listEvents expands recurring masters and passes occurrence proxies to formatEvent. extractRRuleFromItem(item) only reads item.recurrenceInfo, but on occurrence proxies the recurrence lives on item.parentItem.recurrenceInfo -- expanded occurrences will show recurrence: null and isRecurring: false. (item.parentItem || item).recurrenceInfo fixes it.
• extractRRuleFromItem strips the RRULE: prefix but doesn't trim the trailing CRLF that CalRecurrenceRule.icalString sometimes appends.
• RRULE normalization is thin -- leading/trailing whitespace, lowercase rrule: prefix (currently produces invalid RRULE:rrule:...), whitespace-only input.
• SECONDLY / MINUTELY parse via XPCOM but generate no occurrences. Reject upfront with a clear error rather than save a silently-broken event.
• updateEvent schema documents clearing on empty string but the implementation/PR body also accepts null. Worth aligning.
• setRecurrenceOnItem creates a fresh recurrenceInfo, discarding existing EXDATEs / modified exceptions. Probably fine semantically (replace rule = fresh series) but worth a note in the tool description.
• A few RRULE tests would help -- COUNT vs UNTIL, BYDAY=MO,TU, BYMONTHDAY=15, prefix/no-prefix. See test/empty-trash-compat.test.cjs for the pattern.

Holding #95 until this lands since it stacks on top.

[TKasperczyk]

Thanks @realitix. Still need the items from the review above plus a rebase onto current main (showAs/categories/onlineMeeting landed since). The one that's a real bug rather than polish:

listEvents expands recurring masters and hands occurrence proxies to formatEvent, but extractRRuleFromItem only reads item.recurrenceInfo -- on an occurrence proxy that's null, the rule lives on item.parentItem.recurrenceInfo. So recurrence/isRecurring come back null/false on every expanded occurrence, which is the feature's main path. (item.parentItem || item).recurrenceInfo fixes it.

The rest: trim the trailing CRLF off the icalString, tighten RRULE normalization (a lowercase rrule: prefix + no trim currently yields RRULE:rrule:...), reject SECONDLY/MINUTELY upfront (they parse but generate no occurrences), align null-vs-empty-string clearing between schema and impl, note that setRecurrenceOnItem discards existing EXDATEs/exceptions, and a few RRULE tests. Holding #95 until this lands.