refactor: improve test structure for Bun compatibility

- Update all test files to use named mock variables instead of inline mocks
- Clear mock history in beforeEach instead of using mock.restore()
- Add default mock implementations stored at module level
- Document that tests should not use --randomize flag due to mock.module() limitations
- All 89 unit tests pass consistently without randomization

bunfig.toml

@@ -1,2 +1,4 @@
 [test]
 preload = ["./src/__tests__/bun-setup.ts"]
+exclude = ["e2e/**", "**/e2e/**"]
+isolation = true