[Resources] Implement participation endpoint to update resource partStat

[vttranlina]

Epic: #251
We need to implement a new REST API route to allow updating the participation status (ACCEPTED / DECLINED) of a resource.

How

Endpoint format:

/calendar/api/resources/{resourceId}/{eventId}/participation?status={partStat}&referrer=email

• resourceId: Mongo collection resources._id
• eventId: sabredav event identifier (e.g. sabredav-70d3d28f-...)
• partStat: ACCEPTED or DECLINED
• referrer: fixed value = email

Expected response:
TODO: Should be aligned with legacy OpenPaas behavior.

Route implementation

• Create new route class in calendar-rest-api module, Suggested name: ResourceParticipationRoute.
• Validation logic:
  • Ensure both authentication and authorization.
  • [authorization] Check resources.administrators field: Only users whose openpaasId is listed in this collection are authorized.
    Example:

[
  {"_id": ObjectId("68a5850f0873000079ba2ce1"), "id": "68a584caa5eb6f005001c251", "objectType": "user"},
  {"_id": ObjectId("68a58a300873000079ba2cf6"), "id": "68a584caa5eb6f005001c257", "objectType": "user"}
]

Update to SabreDAV

• Update partstat using technical user token.
  • ⚠️ Note: even if a user is listed as administrator in the resource, they cannot directly update. Only the technical user token works.
  • authenticated client (implemented): CardDavClient#authenticatedClientByToken
    
• Add new method in CalDavEventRepository:

public Mono<CalendarEventReportResponse> updateResourcePartStat(
    Username resourceUsername,
    OpenPaaSId resourceId,
    String eventId,
    PartStat partStat
)

• Logic is similar to existing updatePartStat(...) but with one difference: Since we already have the eventId (part of eventPath), there is no need to query Dav by UID to get eventPath (via client.calendarReportByUid)
• Example request

curl --location --request PUT \
'http://localhost:8001/calendars/68a5850f0873000079ba2ce0/68a5850f0873000079ba2ce0/sabredav-70d3d28f-7cfe-43f5-afad-1d49271d366d.ics' \
--header 'TwakeCalendarToken: 73228f19-fc38-4e9f-a5f7-4266ba62d663' \
--header 'Content-Type: text/plain' \
--data-raw 'BEGIN:VCALENDAR
...

END:VCALENDAR'

• SabreDAV responds with HTTP 204 when update succeeds. (a message is published to exchange resource:calendar:event:created)