The org.eclipse.core.runtime.contentTypes extension point was deprecated
in 2018 and is being removed. Replace with its canonical successor
org.eclipse.core.contenttype.contentTypes in the four test fixture
plugin.xml files. The child element schema is identical, so this is a
mechanical id swap with no behavioral change.

These fixtures provide negative-case coverage (missing names, invalid
describers, empty file-extensions, missing alias targets) for the
content-type registry tests in org.eclipse.core.tests.resources.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>