[browser][coreCLR] fix streaming instatiation for NodeJS

[pavelsavara]

• use real Response class when possible

[dotnet-policy-service]

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[Copilot]

Pull request overview

This PR fixes streaming instantiation for Node.js by introducing a responseLike helper function that uses the real Response class when available, and provides appropriate Content-Type headers for assets to enable proper WebAssembly streaming.

Changes:

• Introduced a responseLike function that uses the native Response constructor when available, with proper fallback for environments without it
• Added expectedContentType parameter to fetchLike and thread it through to responses for proper WASM Content-Type headers
• Created behaviorToContentTypeMap to map asset behaviors to their MIME types
• Renamed runtimeToBlazorAssetTypeMap to behaviorToBlazorAssetTypeMap for better naming consistency

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/native/corehost/browserhost/loader/polyfills.ts	Added responseLike helper function, updated fetchLike to accept and use expectedContentType, refactored response creation to use the helper
src/native/corehost/browserhost/loader/assets.ts	Added behaviorToContentTypeMap, renamed asset type mapping for consistency, updated loadResourceFetch to use content types and responseLike

[pavelsavara]

/ba-g known issues