Enable errcheck and staticcheck linters

[tomqin93]

Is your feature request related to a problem? Please describe.

During the Go 1.25.5 upgrade (#1360), golangci-lint was upgraded from v1.x to v2.x. The new version has stricter default linters enabled (errcheck and staticcheck) that were not enforced before, resulting in 52 new lint errors:

• errcheck (36 issues): Unchecked error return values
• staticcheck (16 issues): Various static analysis suggestions

These linters are currently disabled to keep the Go version upgrade PR focused.

Describe the solution you'd like

Enable errcheck and staticcheck linters by removing them from the disable list in .golangci.yml and fixing all 52 issues across the codebase.

Files affected:

• cmd/httpstub/powershellstub/powershellstub.go
• cmd/httpstub/sshstub/sshstub.go
• pkg/deviceshifu/deviceshifubase/deviceshifubase.go
• pkg/deviceshifu/deviceshifuhttp/deviceshifuhttp.go
• pkg/deviceshifu/deviceshifulwm2m/deviceshifulwm2m.go
• pkg/deviceshifu/deviceshifumqtt/deviceshifumqtt.go
• pkg/deviceshifu/deviceshifusocket/deviceshifusocket.go
• pkg/deviceshifu/deviceshifutcp/deviceshifutcp.go
• pkg/gateway/lwm2m/client/lwm2m.go
• pkg/telemetryservice/sql/mysql/mysql.go
• pkg/telemetryservice/sql/sqlserver/sqlserver.go
• pkg/telemetryservice/sql/tdengine/tdengine.go
• Various test files

Describe alternatives you've considered

• Excluding specific paths (examples/, test files) - partially implemented
• Using nolint directives - too verbose

Additional context

Reference: .golangci.yml has a TODO comment pointing to this issue.

[tomqin93]

@claude work on this issue