[ENHANCEMENT] Dispose view listeners and ignore stale views when the webview is replaced #2205

Problem (one or two sentences)

When the sidebar view is replaced, the old view keeps its listeners. A stale view can still dispatch messages and refresh provider state. This work comes from the review of PR #1715.

Context (who is affected and when)

ClineProvider.resolveWebviewView pushes the message, visibility, configuration and dispose listeners into long-lived lists. It does not dispose them when VS Code resolves a new view. A disposed view can also clear listeners that belong to the new view.

A resolve that is still waiting for its HTML can finish after the provider or view is gone. It then installs listeners on a view that no longer exists.

This is a prerequisite for the heartbeat watchdog in PR #1715. The watchdog needs a replaced view to stop refreshing the provider-wide heartbeat.

Desired behavior (conceptual, not technical)

1. Keep the subscriptions of the current view in one list. Dispose that list when a different view replaces it, and when the view is disposed.
2. Ignore events from a view that is no longer the current view. This applies to the message, visibility, configuration and dispose listeners.
3. Stop a resolve that finishes after disposal or replacement. It must not assign HTML or install listeners.
4. Clear this.view when the sidebar view is disposed.
5. Add a test for each stale-resolve guard. This includes the guard after await this.getState(), which no test reaches today.

Constraints / preferences (optional)

• Do not add heartbeat or watchdog code. PR fix(webview): detect dead webview renderer via heartbeat and auto-reload #1715 keeps that work.
• Keep the change small. PR fix(webview): detect dead webview renderer via heartbeat and auto-reload #1715 rebases on this change after it merges.
• Do not use as any in the new tests. Comment every as unknown as cast, as AGENTS.md asks.

Request checklist

• I searched the existing Issues and Discussions for duplicates
• This describes a specific problem with clear context and impact