docs: update cloud run connection docs (#2320)

## Description

Partially fixes
https://github.com/googleapis/mcp-toolbox-sdk-python/issues/496

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🛠️ Fixes #<issue_number_goes_here>

docs/en/how-to/deploy_toolbox.md

@@ -207,6 +207,7 @@ You can connect to Toolbox Cloud Run instances directly through the SDK.
 {{< tab header="Python" lang="python" >}}
 import asyncio
 from toolbox_core import ToolboxClient, auth_methods
+from toolbox_core.protocol import Protocol
 
 # Replace with the Cloud Run service URL generated in the previous step
 URL = "https://cloud-run-url.app"
@@ -217,6 +218,7 @@ async def main():
   async with ToolboxClient(
       URL,
       client_headers={"Authorization": auth_token_provider},
+      protocol=Protocol.TOOLBOX,
   ) as toolbox:
     toolset = await toolbox.load_toolset()
     # ...
@@ -281,3 +283,5 @@ contain the specific error message needed to diagnose the problem.
   Manager, it means the Toolbox service account is missing permissions.
   - Ensure the `toolbox-identity` service account has the **Secret Manager
       Secret Accessor** (`roles/secretmanager.secretAccessor`) IAM role.
+
+- **Cloud Run Connections via IAP:** Currently we do not support Cloud Run connections via [IAP](https://docs.cloud.google.com/iap/docs/concepts-overview). Please disable IAP if you are using it.