Port upstream extensionless image content type fix

[Renakoni]

Upstream

Ports marktext/marktext#4857: marktext/marktext#4857

Android relevance

Android uses Muya image rendering inside the WebView editor. Extensionless remote badge URLs such as https://img.shields.io/badge/example-blue go through the same getImageSrc / loadImage path, so rejecting image/svg+xml;charset=utf-8 or treating an unreadable cross-origin HEAD as "not an image" breaks visible editor content on mobile too.

Changes

• Parse the MIME type portion of Content-Type before matching, so image/svg+xml;charset=utf-8 is accepted.
• Return null from checkImageContentType() when the type cannot be determined, rather than collapsing network/CORS/CSP failures into false.
• Skip guaranteed-unreadable cross-origin HEAD checks and allow the real <img> load to decide.
• Keep rejecting URLs only when a same-origin HEAD positively reports a non-image type.
• Add focused tests for charset-bearing image MIME types, non-image MIME types, non-200/failed HEAD checks, cross-origin skip behavior, and the final loadImage fallback.

Verification

• pnpm --dir third_party/muya exec vitest run src/utils/__tests__/image.spec.ts
• pnpm build