fixes #1989

• I have read the Contribution Guidelines
• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

• Prevent Table.Expand() from distributing extra width into fixed-width columns.
• Add a unit test that verifies a fixed-width column remains stable when expanding.

Testing

• dotnet test src/Spectre.Console.Tests/Spectre.Console.Tests.csproj -c Release --filter FullyQualifiedName~TableTests

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