fix: use tier=1 int instead of RagManagedDbConfig.Tier.BASIC enum

The Tier enum is not available in the installed SDK version.
Use integer value 1 (BASIC) directly instead.

agents/rag/setup_corpus.py

@@ -23,6 +23,10 @@ GCS_SOURCE = os.environ.get(
     f"gs://{PROJECT_ID}-agent-staging/rag-docs/",
 )
 
+# RagManagedDbConfig.Tier enum may not exist in older SDK versions.
+# tier=1 == BASIC, tier=2 == ENTERPRISE
+_TIER_BASIC = 1
+
 
 def ensure_serverless_engine_config() -> None:
     """
@@ -76,10 +80,9 @@ def get_or_create_corpus() -> RagCorpus:
     print(f"Creating corpus '{CORPUS_DISPLAY_NAME}' in {LOCATION} (serverless)...")
     corpus = rag.create_corpus(
         display_name=CORPUS_DISPLAY_NAME,
-        # Explicitly request serverless / basic managed DB tier
         backend_config=rag.RagVectorDbConfig(
             rag_managed_db=rag.RagManagedDbConfig(
-                tier=rag.RagManagedDbConfig.Tier.BASIC,
+                tier=_TIER_BASIC,  # 1 = BASIC (serverless), 2 = ENTERPRISE
             )
         ),
     )