Agentic mode hardcoding

[Mahesh-MSFT]

Documentation says that

When you use agentic authentication, direct messaging in Agents Playground isn't currently supported. You must test through custom activities instead.

However, the code seems have hardcoded to be agentic only. host_agent_server.py has self.auth_handler_name = "AGENTIC" hardcoded.

I needed to change it to -

# Use agentic auth only if enabled, otherwise anonymous
        use_agentic_auth = environ.get("USE_AGENTIC_AUTH", "false").lower() == "true"
        self.auth_handler_name = "AGENTIC" if use_agentic_auth else None

and

handler = [self.auth_handler_name] if self.auth_handler_name else []

to make the code work.