[Remote Agents] Support AgentCards Behind Auth

[adamfweidman]

Handle cases where the initial AgentCard fetch requires authentication (gatekeeper auth).

Requirements:

• Allow users to define "bootstrap" credentials in their local AgentDefinition (markdown frontmatter).
• Apply these credentials to the initial discovery request used to fetch the AgentCard.
• Gracefully handle 401/403 errors during discovery by notifying the user of missing configuration.