style: codebase consistency sweep — JSDoc @returns backticks + != null idiom (#2009)

style: codebase consistency sweep — JSDoc @returns backticks + != null idiom (#2009)

Behavior-preserving codebase style-consistency sweep. Closes #1966.

- Backtick 49 JSDoc @returns boolean literals across 17 files (lowercase True/False).
- Align !== undefined -> != null on number|undefined guards in OCPP20IncomingRequestService.ts (13 tokens); conditional-spread inclusion guards preserved as !== undefined.
- Extend != null idiom to OCPP20VariableManager min/max ternaries + presence guards and the negated EVSE guard.
- Fix OCPP20AuthAdapter.validateConfiguration @returns (synchronous boolean; was falsely 'Promise resolving to').

Gap C (String(previousRequestId) -> .toString()) dropped: premise false — operand is number|undefined, .toString() would fail typecheck (TS18048); String() is the only behavior-preserving form.

All gates green (format/typecheck/lint/build/test, fail=0). Reviewed across multiple multi-agent rounds with cross-validation.