chore(BA-5978): address review feedback

* Rename test class TestBackendAIModelOverride -> TestBackendAISchemaOverride
  to match the BackendAISchema terminology (#3224372077).
* Rename news fragment from 11514.fix.md to 11514.enhance.md — the change
  introduces new infrastructure rather than fixing a bug (#3206461984).

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

Author: jopemachine

changes/11514.fix.md changes/11514.enhance.mdchanges/11514.fix.md renamed to changes/11514.enhance.md

@@ -15,7 +15,7 @@ class _PayloadTestModel(BackendAISchema):
     count: int
 
 
-class TestBackendAIModelOverride:
+class TestBackendAISchemaOverride:
     """``BackendAISchema`` subclasses get the auto-conversion override
     just by inheriting — no per-call-site ``try / except``."""