spa: Fix TestURLToRelAcceptsSafeRelativePath for Windows cross-platform compatibility

[nikunjkumar05]

Summary

This PR fixes TestURLToRelAcceptsSafeRelativePath test failure on Windows by making the test expectation OS-aware using runtime.GOOS.

Changes

• Fixed backend/pkg/spa/pathSafety_internal_test.go to handle Windows path separator conversion
• Added runtime import for OS detection
• Made test expect "headlamp\\assets\\main.js" on Windows and "headlamp/assets/main.js" on Linux/macOS

Steps to Test

1. Run go test -count=1 ./pkg/spa on Windows
2. Verify TestURLToRelAcceptsSafeRelativePath passes

Notes for the Reviewer

• This is a test-only change - no production code modified
• The urlToRel function correctly produces OS-specific paths via filepath.FromSlash() - the test expectation was written for Linux/macOS format only

[kubernetes-prow]

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This pull-request has been approved by: nikunjkumar05
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[Copilot]

Pull request overview

Updates the SPA path-safety test to accept OS-specific path separators.

Changes:

• Adds Windows OS detection.
• Uses backslashes in the Windows expectation.

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[illume]

Thanks for the contribution.

The backend test job in CI is failing. Run cd backend && go test ./... to reproduce the errors locally.

How to run the backend tests

Run cd backend && go test ./... to see all failures. Fix the failing tests and commit the result.

[nikunjkumar05]

@illume thanks for review. Resolved the error.