Add configure connection test command

[ewega]

Summary

Add a configure connection test subcommand to re-test an existing connection by ID. Useful for verifying a connection after token rotation or network changes.

Design

gh devlake configure connection test --plugin gh-copilot --id 2 gh devlake configure connection test # interactive: pick from discovered connections

Changes

• Add test subcommand under configure connection
• Call POST /plugins/{plugin}/connections/{id}/test
• Display detailed test result (success/failure message)
• Client already has TestSavedConnection method

Acceptance Criteria

• Tests existing connection by plugin + ID
• Interactive mode lists connections and lets user pick
• Shows clear success/failure message
• Returns non-zero exit code on failure