I think there are two aspects to this:

1. In general, making systematically sure that all errors are being surfaced as they are being entered is the focus of T358659: Client side form validation.
2. That field in particular is somewhat of a makeshift component which we hopefully replace with the outcome of T345291: [EPIC] MultiselectLookup: Add new component. Then, adding proper validation to that should be easier and not duplicate work.

I think this task could be rephrased to "Namespaces selector allows invalid namespaces". It seems this task is a fair children of T358659, since Namespaces will require specific treatment anyways, thanks for the linking Michael.
Also we could try to keep the "draft" edit summary content so when the user gets a validation error (and fixes it), on the next submit the drafted summary is there. What do. you think? @JFernandez-WMF @KStoller-WMF

In T367829#9913010, @Sgs wrote:

Also we could try to keep the "draft" edit summary content so when the user gets a validation error (and fixes it), on the next submit the drafted summary is there. What do. you think? @JFernandez-WMF @KStoller-WMF

Agreed, this is the current UX expectation for other wiki edit summaries.
Should we include that work in this task, or create a new task?