Extension IDs in extensionDependencies and extensionPack must use lowercase letters per marketplace requirements. Previously, uppercase IDs would pass validation and fail later during publishing. Additionally, this PR warns developers when using the deprecated github.copilot extension.

Changes

• src/validation.ts: Added validateExtensionDependencies() to check extension IDs are lowercase
• src/package.ts: Integrated validation into validateManifestForPackaging() for both extensionDependencies and extensionPack fields, plus added deprecation warning for github.copilot
• Tests: Added unit and integration tests covering valid/invalid scenarios and deprecation warning

Validation Behavior

// ✅ Valid - all lowercase
{
  "extensionDependencies": ["microsoft.vscode-eslint", "esbenp.prettier-vscode"]
}

// ❌ Invalid - uppercase letters
{
  "extensionDependencies": ["Microsoft.vscode-eslint"]  
}
// Error: The extension IDs in "extensionDependencies" must use lowercase letters only. 
//        Invalid IDs: "Microsoft.vscode-eslint".

Deprecation Warning

When github.copilot is detected in either extensionDependencies or extensionPack, a warning is displayed:

⚠ The "github.copilot" extension is being deprecated. Please use "github.copilot-chat" instead.

The validation only checks case; it does not validate ID format or structure.

[!WARNING]

Comments on the Issue (you are @copilot in this section)

• Fixes Validate extension dependencies are using lower case letters #1211

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