refactor: eliminate all non-null assertion suppressions with proper null guards

refactor: eliminate all non-null assertion suppressions with proper null guards

Remove 104 eslint-disable-next-line @typescript-eslint/no-non-null-assertion
suppressions across 23 files by replacing non-null assertions (!) with
semantically correct alternatives: local variable extraction with null
guards, optional chaining (?.), nullish coalescing (??) with proper
default constants, and error logging for invalid states.

Key improvements:
- Use existing DEFAULT_POOL_MAX_SIZE, DEFAULT_POOL_MIN_SIZE, and
  DEFAULT_ELEMENTS_PER_WORKER constants instead of hardcoded fallbacks
- Add proper error logging for powerDivider undefined/invalid states
  instead of silently computing wrong values
- Add connectorId null guard with throw in handleMeterValues instead of
  silently defaulting to connector 0
- Ensure .finally() always sends a response to prevent caller hangs
- Align variable naming with codebase conventions (connectorStatus,
  templateStatistics, commandStatisticsData, entryStatisticsData)