Skip used handles when assigning one from the handle seed

[sommcz]

Description

What failed: reading an R12 DXF with $HANDSEED 0 threw ArgumentException: ... same key ... Key: 0. AddCadObject assigned the seed as the new handle, and handle 0 is the document's own.

Fix: CadDocument.AddCadObject skips handles already in use before assigning one from the seed. With a valid seed nothing changes.

Tasks done in this PR

• Fix in CadDocument.AddCadObject.
• Regression test DxfHandleSeedTests.ReadR12WithZeroHandleSeed: fails on master, passes with the fix.
• Existing tests unaffected: full suite gives the same results as on master.

Related Issues / Pull Requests

• Fixes DxfReader.Read throws ArgumentException: ... same key ... Key: 0 for R12 files with $HANDSEED 0 #1266

Notes for reviewer

• ArcTests.CreateFromBulgeTest and ArcTests.GetCenter fail on master too in our environment (Linux, .NET 10 SDK); unrelated.
• Also checked on 600 real DXF files: the affected ones now read, the rest give identical output.