Implement coordinator pause notification payload builder

Implement PauseNotificationBuilder as a class with only static methods (no instance state) to keep it stateless and easily testable without construction. Localisation strings should be resolved via Flutter's AppLocalizations (generated from .arb files); the builder receives a BuildContext or an AppLocalizations instance as a parameter to stay testable without a widget tree. Truncation logic: compute the maximum allowed length for mentorName as maxBodyLength - templateLengthWithEmptyName - 1 (for ellipsis), then truncate mentorName before interpolation. This avoids truncating in the middle of multi-byte Unicode characters — use String.characters from the characters package for grapheme-cluster-safe truncation.

Do not truncate in the middle of words if possible: prefer truncating at the last word boundary before the limit. Date formatting: use DateFormat.yMMMd(locale) from the intl package. Locale string should be derived from the organisation's configured locale stored in Riverpod state, not from the device locale, to ensure notifications match the organisation's language even on devices with different system language.

Unit tests using flutter_test. All tests are synchronous. Required test cases: (1) standard input with mentorName and returnDate produces correct interpolated title and body, (2) returnDate null uses body_no_date template, (3) very long mentorName (>100 chars) triggers truncation and body remains ≤ 240 chars, (4) very long mentorName where even truncated version with ellipsis does not fit triggers further truncation correctly, (5) toFcmPayloadMap() on returned payload contains required FCM keys, (6) returned recipientType is PauseNotificationRecipientType.coordinator. Use test fixtures for localisation strings rather than loading real .arb files — inject strings via a TestLocalisationDelegate or simple map.