test(07-01): Wave 0 xfail stubs for D-01..D-16 (13 new stubs)

- Create backend/tests/test_ai_providers.py with 7 stubs: test_generic_openai_json_mode, test_anthropic_structured_output, test_get_provider_typed, test_client_singleton, test_context_chars_truncation, test_smart_truncation, test_gemini_fallback_to_parse_classification
- Create backend/tests/test_ai_config.py with 2 stubs: test_load_provider_config, test_api_key_encrypt_decrypt
- Create backend/tests/test_admin_ai_config.py with 2 stubs: test_get_never_returns_key, test_put_writes_active_provider
- Create backend/tests/test_document_tasks.py with 2 stubs: test_retry_backoff, test_exhaustion_sets_failed_status
- Append test_reclassify_requeues_celery xfail stub to test_documents.py
- D-14 regression guard: grep confirms 3 host-gateway entries (>=2 required)

backend/tests/test_documents.py

@@ -923,3 +923,13 @@ async def test_delete_cloud_remove_only(async_client, auth_user, db_session):
     # DB row removed
     deleted = await db_session.get(Document, doc_id)
     assert deleted is None
+
+
+# ---------------------------------------------------------------------------
+# Phase 7 Wave 0 xfail stub — D-11 re-classify endpoint
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.xfail(strict=False, reason="Wave 0 stub — promoted in Plan 07-04")
+async def test_reclassify_requeues_celery():
+    pytest.xfail("not implemented yet — Plan 07-04")