fix(webview): isolate parallel provider view state

Add view-local state loading and syncing for parallel ClineProvider instances,
with safe initialization and configuration listener handling for test/shim
environments.

Key changes:
- Add viewId property for unique provider instance identification
- Implement viewLocalState buffer to isolate mode, profile, and apiConfig per view
- Load view-local state after provider dependencies are initialized
- Merge viewLocalState in getState() with local apiConfiguration taking precedence
- Clear local overrides when saveViewState receives undefined or null values
- Sync mode/profile changes through saveViewState(), loadViewState(), and profile activation
- Preserve restored task mode in both global state and viewLocalState
- Guard global configuration listener setup when VS Code workspace events are unavailable
- Keep sticky-mode task restore compatible with view-local state isolation

Tests:
- Add 22 parallel mode cases covering viewId uniqueness, state isolation,
  save/load behavior, merge precedence, local override clearing, config listener
  handling, mode switching, profile activation, and multi-instance isolation
- Verify sticky-mode restore compatibility

Author: easonliang28