Handle duplicate NZB segments with fallback

[arubi9]

Summary

• preserve NZB segment numbers and collapse duplicate numbers into a single logical segment with ordered fallback candidates
• retry duplicate segment candidates for STAT, HEAD, ARTICLE, BODY, streaming, queue prechecks, and health checks before treating a segment as missing
• add focused regression tests for duplicate-segment parsing and fallback streaming/check behavior

Validation

• dotnet build backend/NzbWebDAV.csproj
• dotnet test backend.Tests/backend.Tests.csproj

Closes #252.

[arubi9]

If some can provide feedback on code changes, that would be great.

[Root-Core]

This looks like an unnecessarily large amount of code to solve this issue, so I skipped the review.

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I think it could be solved much more easily: first, parse the candidates into a map.
If one candidate fails, simply try the next one.
You could also store the ID of the working candidates somewhere if it isn't the first.