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

.github/workflows/ci-backend.yml

@@ -32,7 +32,7 @@ jobs:
       - name: install go
         uses: actions/setup-go@v5
         with:
-          go-version: "1.23"
+          go-version: "1.25"
           cache-dependency-path: backend
 
       - name: test and build backend
@@ -53,15 +53,15 @@ jobs:
           TZ: "America/Chicago"
 
       - name: golangci-lint
-        uses: golangci/golangci-lint-action@v6
+        uses: golangci/golangci-lint-action@v7
         with:
-          version: "v1.64.7"
+          version: "v2.6.0"
           working-directory: backend/app
 
       - name: golangci-lint on example directory
-        uses: golangci/golangci-lint-action@v6
+        uses: golangci/golangci-lint-action@v7
         with:
-          version: "v1.64.7"
+          version: "v2.6.0"
           args: --config ../../.golangci.yml
           working-directory: backend/_example/memory_store