Concourse managed server plugin race condition

[javierlores]

Summary

• Add retry logic with exponential backoff when installing plugins to handle cases where the management server isn't
  ready immediately after server start

Problem

When ClientServerTest starts a server and immediately calls installPlugin(), the management server may not be ready yet, causing the error:
Could not connect to the management server. Please check that Concourse Server is running.

Solution

Added a retry loop in ManagedConcourseServer.installPlugin() that:

• Retries up to 10 times when the management server connection fails
• Uses exponential backoff (500ms → 1s → 2s → 4s → 5s max)
• Preserves original error behavior for non-transient failures