Move interface field type tests to their own class

The tests for unions and interfaces used only by interface fields made
SchemaClassScannerTest the largest test file, so they now live in
SchemaClassScannerInterfaceFieldTest, like SchemaClassScannerDirectiveTest.

The test for a union on the field of a parent interface is dropped. The
scanner finds that interface through the object's own implements clause,
the same path as the test for interfaces discovered later in the scan.
The ordering tests no longer run a query, since building the schema is
what they check. A new test covers a union that is only found through a
member of another such union, which needs a second pass of the step.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: oryan-block