Add Language and Dir Fields to HookConfig

Part of #7435 — Multi-Language Hook Support

User Story

As a template author, I want to explicitly specify the language of my hook script and its working directory so that azd handles it correctly even when auto-detection isn't sufficient.

Solution Approach

Modify pkg/ext/models.go:

1. Import language package from pkg/tools/language.
2. Add new fields to HookConfig struct:

   Language language.ScriptLanguage `yaml:"language,omitempty"`
   Dir      string                  `yaml:"dir,omitempty"`

3. Add private field: resolvedLanguage language.ScriptLanguage (set during validate()).
4. Add accessor: func (hc *HookConfig) ScriptLanguage() language.ScriptLanguage.
5. Add helper: func (hc *HookConfig) IsLanguageHook() bool — returns true if resolvedLanguage != ScriptLanguageUnknown.

Important: Do NOT change validate() in this issue — that happens in Issue 4. This issue only adds struct fields and accessor methods.

Modify pkg/ext/models_test.go:

• Test YAML deserialization with language: python and dir: ./scripts.
• Verify existing tests still pass (new fields are omitempty).
• Verify Language field accepts: python, js, ts, dotnet.

Files: pkg/ext/models.go (MODIFY), pkg/ext/models_test.go (MODIFY)

Acceptance Criteria

• HookConfig marshals/unmarshals YAML correctly with new fields
• Existing hook tests continue to pass (backward compatible)
• Language field accepts python, js, ts, dotnet
• Dir field is a string path
• IsLanguageHook() returns false when resolvedLanguage is unset

References

• Refs: Extends [Issue/enhancement] Cross-platform shell commands behavior #3613
• Contributes to: Multi-Language Hook Support: Python, JavaScript, TypeScript, .NET #7435