Tags: Add MDX test cases

[shilman]

Add test cases for MDX handling, which is already supported in the code.

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Greptile Summary

Added test cases to verify MDX tag handling functionality in Storybook's documentation system, ensuring proper indexing and processing of custom tags in MDX files.

• Added code/addons/docs/template/stories/docs2/Tags.mdx demonstrating Meta component with tags property
• Added test case in StoryIndexGenerator.test.ts to verify MDX files with tags are properly indexed
• Added mock data file __mockdata__/src/docs2/Tags.mdx for testing tag handling
• Modified stories-json.test.ts to validate custom tags ('foo', 'bar') are preserved alongside default tags

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[greptile-apps]

LGTM

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