Fix

The VerifySourcelinks test launches hundreds of dotnet-sourcelink processes simultaneously via Parallel.ForEach with no concurrency limit. Under resource contention, some processes exceed the 60-second timeout in ExecuteHelper.ExecuteProcess, get killed, and report false failures — the tool writes "validated" to stdout but gets killed before it can exit cleanly with code 0.

Root Cause Analysis (from #5259 (comment))

1. Parallel.ForEach launches dotnet-sourcelink test --offline for every PDB — potentially hundreds simultaneously
2. Each process has a 60-second timeout (millisecondTimeout: 60000)
3. Under contention, some take >60s to fully exit
4. The tool validates the file and writes "validated" to stdout, but gets killed during shutdown/cleanup
5. The killed process has a non-zero exit code → added to failedFiles
6. Different PDBs fail each time because it depends on which processes run during peak contention

Change

Add new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount } to bound concurrent process launches to the number of CPU cores.

Fixes #5259