fix(intercom): send last_seen_at as unix seconds when upserting contacts

[tevaum]

What

• Send last_seen_at as an integer unix timestamp in createOrUpdateContact, matching the event and updateLastSeenOnEveryEvent paths in the same file
• Add mocked create/update tests asserting the payload Intercom receives

Why

Intercom's contacts API rejects the ISO-8601 string with 400 parameter_invalid
("'last_seen_at' must be a Unix timestamp in seconds (integer), got String"), so every
identify event carrying an email failed to create or update its contact. The payload is
used for both POST /contacts and PUT /contacts/:id, so contact sync fails in both
branches. Observed on 2.8.5 and reproduced on current newjitsu.

[tevaum]

Let me know if a GH issue is also needed. I can create one with details and logs of the specific problem I'm having on my instance.