Add MemoryStore unit tests

## Context

`MemoryStore` in `internal/auth/store_memory.go` is used for testing but lacks its own tests to verify correct behavior.

## Task

Add unit tests for `MemoryStore` covering:

- `Save()` / `Load()` round-trip
- `Delete()` behavior
- `Exists()` returns correct values
- Edge cases (nil token, load when empty)

## Priority

Low

## Effort

Low