fix(sample): Correct text artifact saving in artifact_save_text sample

Copybara import of the project:
--
9529247 by Yongsul Kim <ystory84@gmail.com>:

COPYBARA_INTEGRATE_REVIEW=#936 from ystory:fix/artifact-sample-save 7b0babc
PiperOrigin-RevId: 766220649

Author: ystory

contributing/samples/artifact_save_text/agent.py

@@ -19,8 +19,15 @@
 
 
 async def log_query(tool_context: ToolContext, query: str):
-  """Roll a die with the specified number of sides."""
-  await tool_context.save_artifact('query', types.Part(text=query))
+    """Saves the provided query string as a 'text/plain' artifact named 'query'."""
+    query_bytes = query.encode('utf-8')
+    artifact_part = types.Part(
+      inline_data=types.Blob(
+          mime_type='text/plain',
+          data=query_bytes
+      )
+    )
+    await tool_context.save_artifact('query', artifact_part)
 
 
 root_agent = Agent(