🐛 [RUM-15689] fix view date to use start time instead of update time

[bcaudan]

Motivation

View events were missing an explicit date field, causing them to default to the event collection time rather than the view's actual start time. This led to incorrect timestamps on view updates.

Changes

• Set date to currentView.startTime in emitViewUpdate so every view event carries the correct start timestamp
• Add test assertions verifying the date on initial and subsequent view updates

Test instructions

Looking at the main view process in RUM Explorer, with this change the relative time should stay at 0s even when the view is updated.

Checklist

• Tested locally (playground)
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated related documentation.

[bcaudan]

@codex review