Update go to 1.26, bump dependencies and apply go fix

[paskal]

Maintenance pass, no functional changes.

Go and dependencies

The go directive moves from 1.24.0 to 1.26.0 and the workflow go-version follows; umputun/baseimage:buildgo-latest currently ships Go 1.26.0, so the Docker build is unaffected. Dependencies updated with go get -u ./..., then go mod tidy and go mod vendor. The bulk of the vendor diff comes from go-dockerclient 1.12.3 -> 1.13.2, which replaces the vendored docker/docker tree with moby/moby/client; testify 1.11.1 -> 1.12.1 accounts for the rest.

CI

actions/checkout and actions/setup-go bumped to v7, golangci-lint pinned to v2.12.2.

go fix

go fix ./app/... applied as a separate commit: slices.Contains, sync.WaitGroup.Go, interface{} -> any, for i := range 100, and the removal of omitempty from the two time.Time fields of core.Request. That last one is a no-op on the wire, since encoding/json ignores omitempty for struct values and always emitted both fields; the README snippet of the same struct is updated to match.

Tests pass with -race against both mongo 4.4 and mongo 8, and golangci-lint v2.12.2 reports no issues.

[paskal]

The coveralls check reports a 0.07% decrease, which is arithmetic rather than a coverage regression: go fix removed five statements that the tests were already exercising, and removing covered statements from a tree below 100% lowers the ratio.

The counts from the coverage profiles, master against this branch: app/agent/docker_notif.go 54 -> 51 statements, where the four-statement contains loop became a single slices.Contains call, and app/server/forwarder.go 47 -> 45, where wg.Add(1) and defer wg.Done() disappeared into wg.Go. No new uncovered code is introduced.