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/map.go

@@ -58,7 +58,7 @@ func deepMap(dst, src reflect.Value, visited map[uintptr]*visit, depth int, conf
 			}
 			fieldName := field.Name
 			fieldName = changeInitialCase(fieldName, unicode.ToLower)
-			if v, ok := dstMap[fieldName]; !ok || (isEmptyValue(reflect.ValueOf(v), !config.ShouldNotDereference) || overwrite) {
+			if _, ok := dstMap[fieldName]; !ok || (!isEmptyValue(reflect.ValueOf(src.Field(i).Interface()), !config.ShouldNotDereference) && overwrite) || config.overwriteWithEmptyValue {
 				dstMap[fieldName] = src.Field(i).Interface()
 			}
 		}