Preserve HTTP errors when loading JS models

[fallintoplace]

Summary

• limit network error wrapping to failures thrown by fetch
• preserve unsuccessful HTTP responses as server errors with the status code
• add focused coverage for HTTP and transport failures

Validation

• npm test -- --grep "ModelLoadManager external service errors"
• npm run build
• isolated TypeScript check for the changed source and test

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[Copilot]

Pull request overview

Preserves HTTP failure details when loading JavaScript models through an external service.

Changes:

• Limits network-error wrapping to rejected fetch calls.
• Includes HTTP status codes in response errors.
• Adds focused HTTP and transport failure tests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
sdk/js/src/detail/modelLoadManager.ts	Separates transport failures from unsuccessful HTTP responses.
sdk/js/test/detail/modelLoadManager.externalService.test.ts	Covers HTTP 404 and rejected-fetch behavior.