FEAT: Add availabilityService (#81)

* auto and fallback work

* test fixes

* fixes

* Show model dialog even if there's no more fallback option

* fix tests

* fix failing test

* disable showInDialog for model in settings

* revert package-lock.json

* remove dup line

---------

Co-authored-by: Sehoon Shon <sshon@google.com>

packages/core/src/routing/modelRouterService.ts

@@ -5,10 +5,6 @@
  */
 
 import type { Config } from '../config/config.js';
-import {
-  PREVIEW_GEMINI_MODEL,
-  DEFAULT_GEMINI_MODEL,
-} from '../config/models.js';
 import type {
   RoutingContext,
   RoutingDecision,
@@ -66,23 +62,6 @@ export class ModelRouterService {
         this.config.getBaseLlmClient(),
       );
 
-      // Unified Preview Model Logic:
-      // If the decision is to use 'gemini-2.5-pro' and preview features are enabled,
-      // we attempt to upgrade to 'gemini-3.0-pro' (Preview Model).
-      if (
-        decision.model === DEFAULT_GEMINI_MODEL &&
-        this.config.getPreviewFeatures() &&
-        !decision.metadata.source.includes('override')
-      ) {
-        // We ALWAYS attempt to upgrade to Preview Model here.
-        // If we are in fallback mode, the 'previewModelBypassMode' flag (handled in handler.ts/geminiChat.ts)
-        // will ensure we downgrade to 2.5 Pro for the actual API call if needed.
-        // This allows us to "probe" Preview Model periodically (i.e., every new request tries Preview Model first).
-        decision.model = PREVIEW_GEMINI_MODEL;
-        decision.metadata.source += ' (Preview Model)';
-        decision.metadata.reasoning += ' (Upgraded to Preview Model)';
-      }
-
       const event = new ModelRoutingEvent(
         decision.model,
         decision.metadata.source,