Fix port number preservation in CSS import URI resolution

[devin-ai-integration]

Fix port number preservation in CSS import URI resolution

Summary

Fixes issue #431 where port numbers were being discarded from URIs when resolving relative CSS imports since version 2.7.0. The bug was in the GetBaseUri() method in ImportRuleCssSource.cs, which explicitly set Port = -1 to exclude port numbers when creating base URIs for resolving relative imports.

Changes:

• Removed the Port = -1 assignment in ImportRuleCssSource.GetBaseUri() to preserve port numbers
• Added comprehensive test ItShould_PreservePortNumber_WhenResolvingRelativeImports() to verify the fix

Impact: This enables CSS imports to work correctly when the base stylesheet is served from a non-standard port (e.g., development servers on localhost:8080).

Review & Testing Checklist for Human

• Verify the test demonstrates the actual bug - Run the new test against the original code (before the fix) to confirm it fails
• Test with real CSS imports and port numbers - Set up a development server on a non-standard port and verify CSS imports work end-to-end
• Investigate original intent - Check git history/documentation to understand why port numbers were originally stripped - there may have been a legitimate reason

---

Diagram

graph TD
    A[PreMailer.cs] --> B[LinkTagCssSource.cs]
    B --> C[ImportRuleCssSource.cs]:::major-edit
    C --> D[GetBaseUri method]:::major-edit
    D --> E[UriBuilder construction]
    
    F[ImportRuleCssSourceTests.cs]:::major-edit --> G[New port preservation test]:::major-edit
    
    H[CSS with @import] --> C
    C --> I[WebDownloader.DownloadString]
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• All 173 existing tests pass, indicating no regressions in current test coverage
• The fix is minimal and focused - only removes the problematic line that strips port numbers
• The new test follows existing patterns in the test suite using Mock
• This change specifically affects relative CSS import resolution, not absolute URLs

Link to Devin run: https://app.devin.ai/sessions/f46a471d948c4b60a7c1d1ee817eccc9
Requested by: @martinnormark

[devin-ai-integration]

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