A2A remote agent client does not load description from agent card on start up

**Describe the bug**
Cold-start problem with remote agent descriptions via the RemoteA2aAgent client in google/adk/agents/remote_a2a_agent.py

1. If you don't supply a description to the remote agent client, then until your messages to the root agent happen to invoke the remote agent, the description will remain empty, relying on the agent name or system instruction alone for delegation to that remote agent. At that point, the description should be loaded by `_ensure_resolved`

2. If you do provide a description, then you won't have a cold-start delegation risk, however then as a developer consuming a remote agent, you will permanently override any agent-card description because L299 checking for a blank description will evaluate False, thereby ignoring the agent-card's description

**Expected behavior**
As with the MCP tool discovery, the remote agent-card should be looked up before communication/task delegation to a remote agent.