Feature big query connector (#15871)

### What problem does this PR solve?

This PR adds Google BigQuery as a first-class data source connector in
RAGFlow.

It enables users to ingest and sync BigQuery data using the same
row-to-document model used by relational database connectors: selected
content columns become document text, metadata columns become document
metadata, an optional ID column provides stable document IDs, and an
optional timestamp column enables cursor-based incremental sync.

The connector supports service-account JSON credentials, table mode,
custom query mode, GoogleSQL queries, cursor-based incremental sync,
deleted-row pruning support, configurable query limits such as
`maximum_bytes_billed`, dry-run validation, batch loading, stable
document IDs, and BigQuery-aware value serialization.

api/apps/services/provider_api_service.py

@@ -337,9 +337,14 @@ async def create_provider_instance(tenant_id: str, provider_id_or_name: str, ins
     api_key_str = ""
     if api_key:
         api_key_str = api_key if isinstance(api_key, str) else json.dumps(api_key)
-    success, msg = await verify_api_key(provider_name, api_key, base_url, region, model_info)
-    if not success:
-        return False, msg
+
+    # Only verify when there are models to probe. Generic providers such as
+    # "OpenAI-API-Compatible" may start empty and receive custom models later.
+    factory_entry = next((f for f in FACTORY_LLM_INFOS if f["name"] == provider_name), None)
+    if (factory_entry and factory_entry.get("llm")) or model_info:
+        success, msg = await verify_api_key(provider_name, api_key, base_url, region, model_info)
+        if not success:
+            return False, msg
 
     extra_fields = {}
     if base_url: