fix: retain RSS entries with blank GUIDs

[vycdev2]

Summary

• Keep RSS items when a present but blank guid would otherwise bypass the existing id/link fallback.
• Select and trim the first nonblank identifier in guid → id → link order.
• Add a regression test for an item with a blank GUID and valid link.

Verification

• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-restore --filter FullyQualifiedName~RssFeedServiceTests --verbosity minimal — passed: 8/8.
• dotnet build --no-restore --verbosity minimal — passed with 0 errors; existing NU1903 SQLitePCLRaw warning.
• dotnet test --no-build --verbosity minimal — passed: 429, skipped: 1, failed: 0.
• git diff --check upstream/develop...HEAD — passed.
• Regression RED check — the new focused test failed on the untouched parser because the item collection was empty.

Risk

• Low: the change is limited to RSS item identifier selection and preserves the existing GUID, ID, then link precedence.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.