[dmt] validate duplicate yaml keys

[diyliv]

Summary

• Use yaml.UnmarshalStrict for module.yaml and Chart.yaml parsing: replaces yaml.Unmarshal with yaml.UnmarshalStrict in CheckDefinitionFile (linter rule), ParseModuleConfigFile, and ParseChartFile.
• Add test for duplicate YAML keys: new TestCheckDefinitionFile_DuplicateKeys reproduces the real bug where descriptions.en was defined twice.
• Affected files: pkg/linters/module/rules/module_yaml.go, internal/module/module.go, pkg/linters/module/rules/module_yaml_test.go

Context

module.yaml with duplicate keys (e.g. en: defined twice under descriptions:) silently passed DMT validation because yaml.Unmarshal drops duplicates by taking the last value. However, the Deckhouse controller uses strict parsing and crashes at runtime:

{"level":"error","msg":"run","error":"start deckhouse controller: ... parse module definition ... yaml: unmarshal errors:\n  line 5: mapping key \"en\" already defined at line 4"}

sigs.k8s.io/yaml UnmarshalStrict was already used elsewhere in the codebase (controller rendering, openapi parsing, values, oss.yaml) but not for module.yaml. This aligns the parsing to strict everywhere.

Example

Before (duplicate keys silently accepted):

name: state-snapshotter
stage: "Preview"
descriptions:
  en: "State snapshotter module"
  en: "Модуль State snapshotter"

→ dmt lint passes ✗

After (duplicate keys caught):

Error: definition-file: Cannot parse file "module.yaml": ...

→ dmt lint fails ✓