Remove the BI event-type override #13

Overview

UwtBITelemetryService.track() accepts an optional eventType in its detail argument (UwtBIEventDetail.eventType). It replaces the standard {namespace}.bi event type for that one event, which causes:

• Inconsistent events: an event can be sent under any type, which breaks the "one BI event type, eventName as a field" model that queries and metrics rely on.
• Ambiguous classification: uwtClassifyTelemetryEvent recognises BI events by the .bi ending, so a custom type ending in .bi can't be told apart from a standard one.
• Wider duplicate-detection key: the key that collapses repeated events includes the event type only because of this option.

Change

• Remove eventType from UwtBIEventDetail, which becomes just scenarioId and feature.
• Always record BI events as {namespace}.bi, and drop the event type from the duplicate-detection key.
• Update DESIGN.md.
• Replace the override tests with one checking that BI events always use {namespace}.bi.

This will be a breaking API change: a caller passing eventType gets a TypeScript error and must remove it.