Switch client result DTOs to discriminated unions

[tiksn]

Summary

• Convert ClientResult and ClientUnitResult from record DTOs to discriminated unions with Success / Failure cases.
• Update helper constructors to return the new union cases.
• Rewrite tests and transport stubs to pattern match on the new API instead of reading Succeeded, Value, and Problem fields.

Testing

• dotnet build .\src\Bridge\Bridge.fsproj -c Release
• dotnet test .\tests\Bridge.Tests\Bridge.Tests.fsproj -c Release
• . pack.ps1