GmailToolset OAuth redirect_uri malformed with appended flowName parameter

[SAMFVH]

Here's the bug report using the provided template:
Describe the bug
GmailToolset OAuth flow generates malformed authorization URLs where the flowName=GeneralOAuthFlow parameter is incorrectly appended to the redirect_uri value instead of being a separate query parameter, causing redirect_uri_mismatch errors in Google OAuth.

To Reproduce
Steps to reproduce the behavior:

1. Install google-adk package
2. Create a GmailToolset with OAuth credentials:

   from google.adk.tools.google_api_tool.google_api_toolsets import GmailToolset
   
   gmail_toolset = GmailToolset(
       client_id="your-client-id",
       client_secret="your-client-secret",
       tool_filter=['gmail_users_drafts_create']
   )

3. Deploy to Google Cloud Run and trigger any Gmail tool function
4. Observe OAuth authorization URL contains malformed redirect_uri: redirect_uri=https://your-url.run.app/dev-ui/ flowName=GeneralOAuthFlow (contains a whitespace)

Expected behavior
OAuth authorization URL should have properly separated query parameters:

redirect_uri=https://your-url.run.app/dev-ui/?flowName=GeneralOAuthFlow 

Note: '?' instead of whitespace or not be there at all?

Error message
Error from Google OAuth:

Error 400: redirect_uri_mismatch
Request details: redirect_uri=https://my-cloud-run-app.us-central1.run.app/dev-ui/ flowName=GeneralOAuthFlow 

Note: There is a space between the URL and "flowName", indicating the flowName is being incorrectly appended to the redirect_uri value instead of being handled separately.

Desktop (please complete the following information):

• OS: macOS
• Python version: Python 3.13.0
• ADK version: 1.2.1 (installed from pip)

Additional context

• Issue occurs in google/adk/auth/auth_handler.py around line 244-250 in the generate_auth_uri() method:
• GmailToolset constructor doesn't accept redirect_uri parameter, causing auth_credential.oauth2.redirect_uri to be None
• The flowName parameter gets incorrectly concatenated to the redirect_uri value instead of being a separate query parameter
• This affects all Google API toolsets using OAuth2 flows
• Registered redirect URI in Google Cloud Console matches the base URL but fails due to the malformed parameter

[peterjliu]

I'm observing this (4)"flowName=GeneralOAuthFlow (contains a whitespace)" in firebase auth. This may prevent firebase auth from working using Google accounts.