Make smart pointer operator bool() explicit

[Copilot]

Non-explicit operator bool() on smart pointers allows silent implicit conversions to integral types, enabling subtle bugs (e.g., pointer arithmetic, unintended overload resolution, comparisons that compile but don't do what's intended).

Changes

• ShadowPtr and OwnerPtr (include/PrettyMemory.h): mark both operator bool() overloads explicit

// Before
operator bool() const { return nullptr != Get(); }

// After
explicit operator bool() const { return nullptr != Get(); }

Contextual conversions (if, while, !, &&, ||, GoogleTest EXPECT_TRUE/EXPECT_FALSE) continue to work — implicit conversion in non-boolean contexts is now rejected by the compiler.