`AgentRegistry.get_mcp_toolset` already derives an `auth_scheme`/`auth_credential`
pair from a registered resource's auth provider binding, but
`get_remote_a2a_agent` ignored bindings entirely. A caller who registered an A2A
agent behind an auth provider had no way to reach it: the returned
`RemoteA2aAgent` sent unauthenticated requests, and the caller could not supply
a credential either.
Accept `auth_scheme`, `auth_credential` and `continue_uri` on
`get_remote_a2a_agent` and forward the pair to `RemoteA2aAgent`. When no scheme
is passed, resolve the agent's `authProviderBinding` into a
`GcpAuthProviderScheme`. The signature and the resolution path mirror
`get_mcp_toolset`, so both registered resource types authenticate the same way.
Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 968076388
Kathy Wu
d6954946df