This commit changes the merge strategy introduced in elastic#55607 and elastic#55982. Instead of overwriting these
fields, we now prevent them from being merged with an exception when a user attempts to
overwrite a field.

As part of this, a more robust validation has been added. The existing validation checked whether
templates (composable and component) were valid on their own, this new validation now checks that
the composite template (mappings/settings/aliases) is valid. This means that when a composable
template is added or updated, we confirm that it is valid with its component pieces. When a
component template is updated we ensure that all composable templates that make use of the component
template continue to be valid before allowing the component template to be updated.

This change also necessitated changes in the tests, however, I have left tests that exercise mapping
merging with nested object fields as `@AwaitsFix`, as we intend to change the behavior soon to allow
merging in a recursive-with-replacement fashion (see: elastic#57393). I have added tests that check the new
disallowing behavior in the meantime.