Migrate golangci-lint to v2 and update Go version (#1965)

* migrate golangci-lint to v2 and update go version

- migrated .golangci.yml to version 2 format
- updated go.mod from 1.23.0 to 1.24
- removed deprecated run.timeout configuration

* update to go 1.25 and baseimage v1.17.0

- updated go.mod to go 1.25
- updated Dockerfile to use buildgo-v1.17.0 (go 1.25.0)
- updated Dockerfile to use app-v1.17.0

* fix flaky tests with proper synchronization

- use assert.Eventually instead of fixed sleep in TestService_Many
- wait for webhook before shutdown in TestMain_WithWebhook
- fixes race conditions exposed by Go 1.25 scheduler changes

* fix data race in MockDest.closed field

- add IsClosed() method with proper locking
- add locking to String() method
- use IsClosed() in tests instead of direct field access
- fixes race condition detected by go test -race

* update example go.mod to go 1.25

* update golangci-lint to v2.6.0 for go 1.25 support

* fix linter issue and update CLAUDE.md

- merge conditional assignment in example accessor/data.go
- add reminder in CLAUDE.md to always test and lint examples before committing

* update example Dockerfile to baseimage v1.17.0 for go 1.25

backend/_example/memory_store/Dockerfile

@@ -1,11 +1,11 @@
-FROM umputun/baseimage:buildgo-v1.14.0 AS build-backend
+FROM umputun/baseimage:buildgo-v1.17.0 AS build-backend
 
 ADD backend /build/backend
 WORKDIR /build/backend/_example/memory_store
 
 RUN go build -o /build/bin/memory_store -ldflags "-X main.revision=0.0.0 -s -w"
 
-FROM umputun/baseimage:app-v1.14.0
+FROM umputun/baseimage:app-v1.17.0
 
 ARG GITHUB_SHA