fix: use correct attendee name in invitation acceptance notifications

https://gitlab.e.foundation/e/infra/backlog/-/issues/4732

Test Cases — Invitation Acceptance Notification (Email) Bug

TC-1: Multiple Attendees — Last Attendee Accepts

Scenario: Meeting with 3 attendees where the last attendee in the list accepts
Preconditions / Setup

• Create a meeting with attendees (in this exact order):
  • Alice alice@example.com — first
  • Bob bob@example.com — second
  • Charlie charlie@example.com — last

Action

• Charlie accepts the invitation

Expected Result

• The acceptance notification email sent to the organizer states:
  “Charlie has accepted your invitation…”

Regression Note (previous bug)

• Previously, the email incorrectly showed “Alice has accepted…” (wrong attendee name).

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TC-2: Single Attendee — Basic Acceptance

Scenario: Meeting with only one attendee
Preconditions / Setup

• Create a meeting with a single attendee:
  • John john@example.com

Action

• John accepts the invitation.

Expected Result

• The acceptance notification email sent to the organizer states:
  “John has accepted your invitation…”