recordings: capture session:transport-gap so a replay shows the gap row (#1381 follow-up) #1444

Follow-up of #1381 / PR #1442.

Session recordings (SessionRecorderManager's channel list, recordedSession, reconstructSlices) do not capture session:transport-gap, nor the transportGaps field of the initial history chunk. A recording of a session that lost a proxy span therefore replays without the durable "not captured" row the user saw.

Fix shape:

• add the channel to the recorded set;
• fold it into the reconstructed runtime's transportGaps;
• pin it with a replay test.

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