fix(tests): resolve 6 test isolation failures in full suite

3 failures in fallback.cliproxyapi-matrix.test.ts:
- Root cause: leaked spyOn(getMainSessionID) in
  tool-execute-before-session-notification.test.ts was never restored,
  poisoning module state for subsequent tests in the same worker
- Added mockRestore() call and _resetModelFallbackForTesting in afterEach

3 failures in background-agent/manager.test.ts:
- Root cause: connected-providers-cache memConnected/memProviderModels
  persisted across test files, making isReachable() skip fallback
  candidates in retry tests
- Added mock.module for connected-providers-cache at file level
- Added _resetMemCacheForTesting export and global beforeEach cleanup

Full suite: 4638/4638 pass, 0 fail

src/features/background-agent/manager.test.ts

@@ -1,5 +1,15 @@
 declare const require: (name: string) => any
-const { describe, test, expect, beforeEach, afterEach, spyOn } = require("bun:test")
+const { describe, test, expect, beforeEach, afterEach, spyOn, mock } = require("bun:test")
+
+mock.module("../../shared/connected-providers-cache", () => ({
+  readConnectedProvidersCache: () => null,
+  readProviderModelsCache: () => null,
+  hasConnectedProvidersCache: () => false,
+  hasProviderModelsCache: () => false,
+  writeProviderModelsCache: () => {},
+  updateConnectedProvidersCache: () => {},
+}))
+
 import { getSessionPromptParams, clearSessionPromptParams } from "../../shared/session-prompt-params-state"
 import { tmpdir } from "node:os"
 import type { PluginInput } from "@opencode-ai/plugin"