fix(adk): set correct URL for remote agent card

[onematchfox]

Addresses issue whereby agent->agent calls are currently failing with errors like this in the UI:

{"id":"call_xx": "name": "transfor_to_agent", "response":{"result": null}}

Agent pods show the following errors:

ERROR:google_adk.google.adk.agents.remote_a2a_agent:Failed to resolve remote A2A agent kagent__NS__my_agent: Failed to resolve AgentCard from URL http://my-agent.kagent:8080/: HTTP Error 405: Failed to fetch agent card from http://my-agent.kagent:8080/: Client error '405 Method Not Allowed' for url 'http://my-agent.kagent:8080/'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/405

I guess this was my bad, and it's related to the upgrade in #816.

[Copilot]

Pull Request Overview

This fix resolves agent-to-agent communication failures by correcting the URL format used for remote agent cards. The issue was causing 405 Method Not Allowed errors when agents attempted to communicate with each other.

• Import the AGENT_CARD_WELL_KNOWN_PATH constant from the remote agent module
• Update the agent card URL construction to append the well-known path to the base URL

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