Apply unpin partial updates and stamp participant message events with the current created_at

[andremion]

Goal

Two fixes needed by the upcoming Android pinned-message e2e tests (the first pin coverage on any platform).

1. Unpin was silently dropped. The Android SDK unpins a message with a partial update set: {pinned: false}. The update handler branched on the value of set.pinned, which is falsy for false, so the unpin never applied and no message.updated event was sent.
2. Participant message events carried a stale template timestamp. The /participant/message route never set the event-level created_at, so every event it broadcast kept the static date from the ws_events.json template. The Android client sorts each batch of near-simultaneous events by event created_at, so a participant pin (dated with the template's old date) was applied before the message.new echo of the message it pinned, and the pin was overwritten.

Related: AND-1328

Implementation

• update_message branches on the presence of the pinned key instead of its value, so pinned: false is applied like pinned: true.
• The /participant/message route stamps the event's created_at with the current timestamp, the same way create_event and create_reaction already do. This affects all participant message events, not only pins, which matches the real backend: an event is dated when it happens.

Testing

• bundle exec rubocop: no offenses.
• Ran the new Android PinnedMessagesTests (4 tests: user pin, user unpin, participant pin, participant unpin) locally against this branch: all green. Against main, only user pin passes.
• Regression: ran the Android ChannelListTests (12 tests) and ReactionsTests (10 tests) locally against this branch: all green. These cover the participant message and reaction flows affected by the timestamp change.
• iOS is unaffected today: its e2e suite has no pin tests, and the timestamp fix only corrects the event date.

[andremion]

Started e2e tests on this branch:

• Android
• iOS