fix: chain root cause in create_analytics_views() startup failure

Persist the init exception as _startup_error and use `raise ... from
self._startup_error` so callers get actionable context instead of a
generic RuntimeError.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: caohy1988

src/google/adk/plugins/bigquery_agent_analytics_plugin.py

@@ -1763,6 +1763,7 @@ def __init__(
     self.location = location
 
     self._started = False
+    self._startup_error: Optional[Exception] = None
     self._is_shutting_down = False
     self._setup_lock = None
     self.client = None
@@ -2141,7 +2142,7 @@ async def create_analytics_views(self) -> None:
     if not self._started:
       raise RuntimeError(
           "Plugin initialization failed; cannot create analytics views."
-      )
+      ) from self._startup_error
     loop = asyncio.get_running_loop()
     await loop.run_in_executor(self._executor, self._create_analytics_views)
 
@@ -2196,6 +2197,7 @@ def __getstate__(self):
     state["offloader"] = None
     state["parser"] = None
     state["_started"] = False
+    state["_startup_error"] = None
     state["_is_shutting_down"] = False
     state["_init_pid"] = 0
     return state
@@ -2224,6 +2226,7 @@ def _reset_runtime_state(self) -> None:
     self.offloader = None
     self.parser = None
     self._started = False
+    self._startup_error = None
     self._is_shutting_down = False
     self._init_pid = os.getpid()
 
@@ -2247,7 +2250,9 @@ async def _ensure_started(self, **kwargs) -> None:
           try:
             await self._lazy_setup(**kwargs)
             self._started = True
+            self._startup_error = None
           except Exception as e:
+            self._startup_error = e
             logger.error("Failed to initialize BigQuery Plugin: %s", e)
 
   @staticmethod

tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py

@@ -4943,5 +4943,10 @@ async def test_create_analytics_views_raises_on_startup_failure(
           dataset_id=DATASET_ID,
           table_id=TABLE_ID,
       )
-      with pytest.raises(RuntimeError, match="Plugin initialization failed"):
+      with pytest.raises(
+          RuntimeError, match="Plugin initialization failed"
+      ) as exc_info:
         await plugin.create_analytics_views()
+      # Root cause should be chained for debuggability
+      assert exc_info.value.__cause__ is not None
+      assert "client boom" in str(exc_info.value.__cause__)