chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/dario.cat/mergo/merge.go

@@ -269,7 +269,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 					if err = deepMerge(dst.Elem(), src.Elem(), visited, depth+1, config); err != nil {
 						return
 					}
-				} else {
+				} else if src.Elem().Kind() != reflect.Struct {
 					if overwriteWithEmptySrc || (overwrite && !src.IsNil()) || dst.IsNil() {
 						dst.Set(src)
 					}