explicitly call EnsurePullability while adding tables

[heavycrystal]

The UI for dynamic table addition should prevent tables that we cannot mirror via CDC [no primary key + replident != full] to be selected. However, we end up using the signal for adding tables sometimes, and currently there is no check in this codepath.

Invoking EnsurePullability explicitly to guard against this.

[CLAassistant]

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