Add command to `coder ping` a workspace

[spikecurtis]

coder ping is useful to diagnose connection problems to workspaces. We should add support to the plugin to invoke it on a workspace.

Scope

Implement a Coder: Ping Workspace command that:

1. Executes coder ping against the currently connected workspace via the CLI.
2. Shows progress using VS Code's progress notification (vscode.window.withProgress) while the command runs.
3. Outputs results in a VS Code terminal — similar to how a build command works. The user can see the live ping output (latency, P2P diagnostics, DERP info, etc.) streaming in real-time.
4. The command should be available in the Command Palette when connected to a workspace.

UX

• Command Palette: Coder: Ping Workspace
• Opens a terminal and runs coder ping <workspace> with live output.
• Progress indicator shown while the command is active.

Follow-up

• ENG-2302 — Proactive slowness detection (prompt users to run this command automatically) and speedtest visualization.