Three fixes for the model selection bugs reported in #11656 and 12+ related issues:

1. Fix loadConfig() init ordering: await isInitialized before checking
   currentProfile, consistent with getSerializedConfig(). Prevents
   returning config: undefined during initialization race.

2. Avoid full config reload on model selection: The config/updateSelectedModel
   handler now updates the model in the cached config in-place instead of
   doing a full reloadConfig(). This eliminates the window where
   selectedModelByRole.chat is null during reload, and prevents the
   configUpdate from core overwriting the GUI's optimistic selection.

3. Guard against undefined config in GUI: handleConfigUpdate now also
   checks !configResult.config alongside configLoadInterrupted, preventing
   EMPTY_CONFIG (with chat: null) from being applied.

Two fixes for the "No chat model selected" error reported in #11656 and
12+ related issues across all providers:

1. Fix loadConfig() init ordering (ConfigHandler.ts): Move
   `await this.isInitialized` before the `!this.currentProfile` check,
   matching the pattern already used in getSerializedConfig(). Without
   this, loadConfig() called during initialization returns
   config: undefined because currentProfile hasn't been set yet.

2. Guard against undefined config in GUI (ParallelListeners.tsx):
   handleConfigUpdate now also checks `!configResult.config` alongside
   configLoadInterrupted, preventing EMPTY_CONFIG (with
   selectedModelByRole.chat: null) from being applied.

The added `!configResult.config` guard in ParallelListeners caused tests
to bail early since MockIdeMessenger returned `config: undefined`.