Language Resolution in HookConfig.validate()

Part of #7435 — Multi-Language Hook Support

User Story

As a template author, I want azd to automatically detect that my seed.py hook is a Python script so I don't have to specify language: python in every hook definition.

Solution Approach

Modify validate() in pkg/ext/models.go. Insert resolveLanguage() step after determining file vs inline, before shell inference:

1. If explicit Language field set → use it, set resolvedLanguage.
2. Else if explicit Shell field set → resolvedLanguage stays Unknown (shell hook).
3. Else if file-based → call language.InferLanguageFromPath(path):
   • Recognized language → set resolvedLanguage.
   • Not recognized → fall through to shell inference (existing behavior).
4. Else → inline script, fall through to shell inference.

Additional logic:

• If resolvedLanguage != Unknown AND location == ScriptLocationInline → return error: "inline scripts are not supported for language hooks (language: %s). Use a file path instead."
• If resolvedLanguage != Unknown → set validated = true, return nil (skip shell inference).
• Update inferScriptTypeFromFilePath() error message to acknowledge language scripts.
• Update ErrUnsupportedScriptType error variable for clarity.

Modify pkg/ext/models_test.go — table-driven tests:

• .py → ScriptLanguagePython
• .js → ScriptLanguageJavaScript
• .ts → ScriptLanguageTypeScript
• .cs → ScriptLanguageDotNet
• .sh → ScriptLanguageUnknown (shell hook)
• .ps1 → ScriptLanguageUnknown
• Explicit language: python with .sh extension → Python (override wins)
• Explicit shell: sh with .py extension → shell hook (shell wins)
• Inline with language: python → error
• No extension, no shell, no language → OS default shell

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

Acceptance Criteria

• All existing hook tests continue to pass (backward compatible)
• Language correctly inferred from .py, .js, .ts, .cs extensions
• Explicit language: field takes precedence over extension
• Explicit shell: field takes precedence over language inference
• Inline scripts rejected for language hooks with clear error
• Shell hooks (.sh, .ps1, inline) completely unaffected

References

• Refs: Extends [Issue/enhancement] Cross-platform shell commands behavior #3613, service hook: unable to determine script type #1579
• Contributes to: Multi-Language Hook Support: Python, JavaScript, TypeScript, .NET #7435