feat: add audience and prompt as configurable for OAuth flows

Merge https://github.com/google/adk-python/pull/2738

Some OAuth servers require audience such as [Jira](https://developer.atlassian.com/cloud/confluence/oauth-2-3lo-apps/). This change allows the configuration of audience and prompt (if it needs to be changed) and adds some tests.

This is for issue [2755](https://github.com/google/adk-python/issues/2755).

Resolves #2755

Happy to provide changes/updates if needed.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2738 from mescanne:oauth-audience-prompt 87ce1100792d9156ada2a004bcfaf2fe5fc69602
PiperOrigin-RevId: 802850034

src/google/adk/auth/auth_credential.py

@@ -79,6 +79,7 @@ class OAuth2Auth(BaseModelWithConfig):
   refresh_token: Optional[str] = None
   expires_at: Optional[int] = None
   expires_in: Optional[int] = None
+  audience: Optional[str] = None
 
 
 class ServiceAccountCredential(BaseModelWithConfig):

src/google/adk/auth/auth_handler.py

@@ -188,9 +188,16 @@ class AuthHandler:
         scope=" ".join(scopes),
         redirect_uri=auth_credential.oauth2.redirect_uri,
     )
+    params = {
+        "access_type": "offline",
+        "prompt": "consent",
+    }
+    if auth_credential.oauth2.audience:
+      params["audience"] = auth_credential.oauth2.audience
     uri, state = client.create_authorization_url(
-        url=authorization_endpoint, access_type="offline", prompt="consent"
+        url=authorization_endpoint, **params
     )
+
     exchanged_auth_credential = auth_credential.model_copy(deep=True)
     exchanged_auth_credential.oauth2.auth_uri = uri
     exchanged_auth_credential.oauth2.state = state

tests/unittests/auth/test_auth_handler.py

@@ -61,7 +61,10 @@ class MockOAuth2Session:
     self.state = state
 
   def create_authorization_url(self, url, **kwargs):
-    return f"{url}?client_id={self.client_id}&scope={self.scope}", "mock_state"
+    params = f"client_id={self.client_id}&scope={self.scope}"
+    if kwargs.get("audience"):
+      params += f"&audience={kwargs.get('audience')}"
+    return f"{url}?{params}", "mock_state"
 
   def fetch_token(
       self,
@@ -225,8 +228,27 @@ class TestGenerateAuthUri:
         "https://example.com/oauth2/authorize"
     )
     assert "client_id=mock_client_id" in result.oauth2.auth_uri
+    assert "audience" not in result.oauth2.auth_uri
     assert result.oauth2.state == "mock_state"
 
+  @patch("google.adk.auth.auth_handler.OAuth2Session", MockOAuth2Session)
+  def test_generate_auth_uri_with_audience_and_prompt(
+      self, openid_auth_scheme, oauth2_credentials
+  ):
+    """Test generating an auth URI with audience and prompt."""
+    oauth2_credentials.oauth2.audience = "test_audience"
+    exchanged = oauth2_credentials.model_copy(deep=True)
+
+    config = AuthConfig(
+        auth_scheme=openid_auth_scheme,
+        raw_auth_credential=oauth2_credentials,
+        exchanged_auth_credential=exchanged,
+    )
+    handler = AuthHandler(config)
+    result = handler.generate_auth_uri()
+
+    assert "audience=test_audience" in result.oauth2.auth_uri
+
   @patch("google.adk.auth.auth_handler.OAuth2Session", MockOAuth2Session)
   def test_generate_auth_uri_openid(
       self, openid_auth_scheme, oauth2_credentials