feat: delegate manual attachment to external terminals

[eitsupi]

#1868 and #1870 provide managed sess preparation, connection references, and explicit terminal binding, but manual attachment still has a gap. When an external terminal has no live session binding, the status-bar attach action can send the native command to vscode-R's global endpoint. In vscode-R-console, that bypasses SessProxy and replaces the proxy-backed transport. PID discovery can also wait indefinitely on an extension-owned pseudoterminal before its frontend gets a chance to recover.

This adds session.registerTerminalIntegration(terminal, { attach(token) }) so the frontend can restore its own transport and bind the accepted connection using the existing APIs:

• While a registered handler is pending, repeated attach requests remain busy even if a live binding already exists. Otherwise a live terminal owner is focused first, or the registered handler runs before PID discovery. Registered terminals are excluded from native PID association and use explicit binding.
• Registration survives disconnects and connection replacement. Disposal, terminal close, and watcher shutdown revoke it and request cancellation; moving focus away also cancels a pending attempt.
• Concurrent attach requests do not submit duplicate work. Success requires a current live terminal binding, and delayed completion preserves a newer choice of another session. A decline, exception, or cancellation never falls back to the native command.

Consumers can prepare through their proxy, observe the matching connection, and call activate(sessionId, { terminal, connectionId, select: false }) before returning true. vscode-R then verifies the binding and selects/focuses the terminal. Consumers retain responsibility for prompt readiness, submission queues, failure explanations, and R code already submitted when cancellation occurs.

This completes the third part of the external session API for the intended 3.2.0 release alongside #1868 and #1870, so consumers can adopt managed sess with a supported manual recovery path. This change is on the TypeScript side and does not change the bundled sess revision or introduce another reinstall/restart transition. External frontends must adopt the hook; pasting a copied native attach command remains outside this API and can still bypass a proxy.