Administration: Sanitize category and tag base to prevent URL encoding issues in permalinks

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/16839

This PR fixes category and tag base sanitization in permalink settings to prevent 404 errors when bases contain spaces or special characters.

When users set a category base with spaces (e.g., "Foo Bar"), WordPress generates URLs like example.com/Foo%20Bar/category, which results in 404 errors because rewrite rules expect example.com/foo-bar/category.

This PR -

• Sanitizes category_base and tag_base using sanitize_title_with_dashes()
• Handles multiple path segments correctly (e.g., "Foo Bar/Sub" → "foo-bar/sub")
• Maintains backward compatibility with existing slugified bases
• Addresses Unicode character handling mentioned in the original ticket
• Added unit tests covering input scenarios

[github-actions]

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