Add configure connection test subcommand

[Copilot]

Adds gh devlake configure connection test to re-test an existing connection by plugin + ID — useful after token rotation or network changes.

Changes

• cmd/configure_connection_test_cmd.go — New test subcommand registered under configureConnectionsCmd:
  • Flag mode: both --plugin and --id provided → validates plugin, calls TestSavedConnection directly
  • Interactive mode: either flag missing → discovers DevLake, lists all connections across plugins, prompts selection
  • Returns non-zero exit code when result.Success == false
• cmd/configure_connection_test_cmd_test.go — Tests for subcommand registration, plugin validation, and flag defaults
• README.md — Documents the new command between the connection list and configure scope sections

Usage

# Non-interactive
gh devlake configure connection test --plugin gh-copilot --id 2

# Interactive: pick from all discovered connections
gh devlake configure connection test

Uses the existing client.TestSavedConnection(plugin, connID) method — no new API surface added.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add configure connection test command</issue_title>
<issue_description>## 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
  </issue_description>

<agent_instructions>v0.3.4 just shipped — commands are now singular (connection, scope, project), --plugin flag exists, and configure connection list is a subcommand of configureConnectionsCmd. Work off the latest main.

Key implementation points:

1. Create a new file cmd/configure_connection_test_cmd.go (NOT _test.go — that's a Go test file) with a test subcommand under configureConnectionsCmd
2. The API method TestSavedConnection(plugin, connID) ALREADY EXISTS in internal/devlake/client.go — use it directly, do NOT add a duplicate
3. Flag mode: --plugin + --id required
4. Interactive mode: list connections (reuse pattern from configure_connection_list.go), let user pick one
5. Display clear success/failure message with connection details
6. Return non-zero exit code on test failure (return an error from RunE)
7. Follow terminal output rules in .github/instructions/terminal-output.instructions.md
8. Update README.md with the new command documentation
9. Add tests in configure_connection_test_cmd_test.go
10. Run go build ./... and go test ./... to verify</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add configure connection test command #10

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[Copilot]

Pull request overview

This PR adds a gh devlake configure connection test subcommand to verify existing DevLake connections after token rotation or network changes. The implementation follows established patterns from the connection list subcommand, supporting both flag-based (non-interactive) and interactive modes.

Changes:

• New test subcommand under configure connection with dual-mode operation (flag mode requires both --plugin and --id, otherwise enters interactive selection)
• Reuses existing client.TestSavedConnection() API method, no new API surface added
• Comprehensive documentation in README.md positioned between connection list and configure scope sections

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
cmd/configure_connection_test_cmd.go	New test subcommand with flag validation, interactive connection selection, and clear success/failure output following terminal output standards
cmd/configure_connection_test_cmd_test.go	Unit tests covering command registration, plugin validation via FindConnectionDef, and flag defaults
README.md	Documentation for the new command including usage examples, flag descriptions, and output samples