Add statSessionFile helper for OpenCode DB virtual paths

[rajbos]

This pull request introduces a new dependency, statSessionFile, to the backend and sync service layers, enabling support for virtual file systems (such as OpenCode DB) by allowing custom file stat implementations. The change replaces direct calls to fs.promises.stat with this injected helper, improving flexibility and testability. All relevant tests and backend initializations are updated to provide the new dependency.

Backend and Service Layer Enhancements:

• Added a new required dependency, statSessionFile, to the BackendFacadeDeps and SyncServiceDeps interfaces, allowing custom file stat implementations for session files. [1] [2]
• Updated BackendFacade and SyncService to use the injected statSessionFile helper instead of direct fs.promises.stat calls, supporting virtual file system scenarios. [1] [2] [3]

Extension and Test Integration:

• Modified the VS Code extension activation to inject statSessionFile from the tokenTracker, ensuring the backend receives the correct implementation.
• Updated all backend-related tests and test helpers to provide a statSessionFile implementation, ensuring compatibility and correctness in test environments. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]