Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.
Import 18 network packet test files and 5 server infrastructure test
files, adapted for main branch APIs: fix config import alias (_config),
remove non-existent DevMode field, use global handlerTable instead of
per-server handlers map, and correct validateToken mock expectations
to include both token and tokenID arguments.
Adds go-sqlmock dependency for database mocking in signserver tests.
