Fix variable frontmatter error - handle both string and object formats for preview image fields

[Copilot]

Problem

The extension was throwing runtime errors when frontmatter fields used variable formats (union types), specifically when a field could be either a string or an object. This commonly occurs with Astro's content schema definitions:

// Astro content.config.ts
cover: z.union([
  z.string(),
  z.object({
    src: z.string(),
    title: z.string().optional(),
  }),
])

Both of these frontmatter formats should be valid:

# String format
cover: some-filename.jpg

# Object format  
cover:
  src: some-filename.jpg
  title: a title for the file

However, the extension was calling .startsWith('http') directly on the field value without type checking, causing errors when the value was an object instead of a string.

Solution

1. Main Fix in PagesParser.ts

Added proper type checking before calling string methods on frontmatter field values:

• ✅ String format: Uses the value directly
• ✅ Object format with src property: Extracts the src value
• ✅ Invalid formats: Safely skips processing without errors
• ✅ Only calls string methods on validated string values

2. Additional Fix in ContentType.ts

Fixed validation logic that was calling .length on potentially non-string/non-array values:

• ✅ Added type checking before accessing .length property
• ✅ Updated return type to reflect actual returned data types

Testing

Created comprehensive tests covering all scenarios:

PagesParser scenarios:

• String format: cover: "image.jpg" ✓
• Object format: cover: { src: "image.jpg", title: "title" } ✓
• External URLs in both formats ✓
• Invalid objects and null values handled safely ✓

ContentType validation scenarios:

• Proper handling of null, undefined, empty values ✓
• Safe handling of objects and numbers without crashes ✓

Result

The extension now gracefully handles variable frontmatter formats without throwing errors or showing error popups. Users can confidently use Astro-style union types and other variable schemas in their frontmatter without encountering runtime errors.

Fixes #958.

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