refactor(model-fallback): fully encapsulate session state in factory closure

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin/fallback.cliproxyapi-matrix.test.ts

@@ -9,7 +9,6 @@ import { createModelFallbackHook } from "../hooks/model-fallback/hook"
 import { createRuntimeFallbackHook } from "../hooks/runtime-fallback"
 import type { RuntimeFallbackPluginInput } from "../hooks/runtime-fallback/types"
 import { _resetForTesting } from "../features/claude-code-session-state"
-import { _resetForTesting as _resetModelFallbackForTesting } from "../hooks/model-fallback/hook"
 import { SessionCategoryRegistry } from "../shared/session-category-registry"
 import * as connectedProvidersCache from "../shared/connected-providers-cache"
 
@@ -369,7 +368,6 @@ function setupConnectedProviderCacheMocks(): void {
 
 afterEach(() => {
   _resetForTesting()
-  _resetModelFallbackForTesting()
   SessionCategoryRegistry.clear()
 })