Migrate to JSpecify annotations

[vLuckyyy]

Description:
Replace existing nullability annotations (e.g., @Nullable, @NotNull, etc.) with standardized JSpecify annotations.
JSpecify provides a unified and well-defined approach to nullness checking across Java tools, improving code clarity and IDE/static analysis support.

Acceptance criteria:

• All non-standard nullability annotations are replaced with JSpecify equivalents.
• Build compiles successfully without warnings.
• Static analysis tools (e.g., IDE, SpotBugs) correctly recognize the new annotations.

[vLuckyyy]

We may return to this topic in the future.