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Display an error message when OAuth authentication fails
Currently, when ADK Web directs a user through an OAuth authentication flow and the flow fails, no error message or guidance is shown to the user. This results in a silent failure where the user is redirected back to ADK Web without any indication of what went wrong or how to resolve it.
I suggest displaying a notification banner at the bottom of the screen when the user is redirected back with an error query parameter in the URL.
Steps to Reproduce
- Create an invalid OAuth configuration by requesting an invalid scope.
- Create an agent that accesses an MCP resource protected by OAuth.
- Run ADK Web.
- Ask the agent a question to trigger the authentication flow.
- The OAuth flow redirects back to ADK Web with an error query parameter.
Expected Behavior ADK Web should display a clear, user-facing error message indicating that the OAuth authentication failed and the reason.
Actual Behavior The user is redirected back silently with no visual feedback or error message shown, making it unclear that the authentication failed.
Additional Context This behavior breaks the user flow and can be confusing, especially for first-time users setting up OAuth-protected MCP resources.
