Add manifest validation and improve error handling

[priyagupta108]

Description:
This fix ensures proper validation of the manifest before processing and improves error handling by falling back to a secondary source for unexpected manifest structures.

Related issue:
#580

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

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Pull Request Overview

This PR improves the manifest validation logic and error handling in the installer module.

• Adds a type guard function (isIToolRelease) to validate manifest objects.
• Improves error handling in getManifest by providing a fallback mechanism to fetch the manifest via URL.

Comments suppressed due to low confidence (2)

src/installer.ts:308

• [nitpick] Consider enhancing the error message to include more context about the failure in manifest validation (e.g., indicating that the fetched manifest does not match the expected structure) to ease troubleshooting.

let errorMessage = 'An unexpected error occurred while fetching the manifest.';

src/installer.ts:278

• [nitpick] Consider renaming 'isIToolRelease' to 'isToolRelease' to better align with naming conventions that avoid prefixed 'I' when used in type guard function names, if consistent with the rest of the codebase.

function isIToolRelease(obj: any): obj is tc.IToolRelease {