1. 1. Handle NamespacePlacementChanged events with empty namespace in DbAbsent state

This change enables the change stream shard targeter to handle `NamespacePlacementChanged` events with empty namespaces (which occur during FCV upgrades/downgrades) when in the DbAbsent state.

1. 1. 1. Key Changes

• *Event Handler Logic*: Added handling for `NamespacePlacementChangedControlEvent` in the DbAbsent state event handler. When placement history is unavailable, the system switches to v1 mode; otherwise, it continues processing.

• *Control Event Filtering*: Updated the config server control event filter to include `NamespacePlacementChanged` events with empty namespaces alongside existing database creation events using an `$or` query.

• *Event Type Registration*: Added `NamespacePlacementChangedControlEvent::opType` to the set of control event types processed on the config server.

• *Validation*: Added assertions to ensure only cluster-level (empty namespace) `NamespacePlacementChanged` events are processed in DbAbsent state.

1. 1. 1. Test Coverage

• Added test cases verifying proper handling of `NamespacePlacementChanged` events with empty namespaces
• Added validation that events with non-empty namespaces are rejected with appropriate error messages
• Updated existing test expectations to include the new event type in control event filters
• Added tests for both placement history available and unavailable scenarios