fix: stabilize slots button bet values

[vycdev2]

Summary

• serialize slots button bet values with the invariant decimal separator
• parse button bet values strictly with invariant culture so locale changes cannot reinterpret an amount
• add regression coverage for comma-decimal cultures and ambiguous comma-formatted values

Verification

• dotnet build — passed: 0 errors (2 existing package-vulnerability warnings)
• dotnet test --no-build --filter 'FullyQualifiedName~SlotsModuleTests|FullyQualifiedName~SlotsServiceTests' --logger 'console;verbosity=minimal' — passed: 23/23
• dotnet test --no-build --logger 'console;verbosity=minimal' — partial: 302 passed, 2 unrelated existing tr-TR cases failed because this runner uses invariant globalization
• git diff --check upstream/develop...HEAD — passed

Risk

• Low — the change only makes the internal Discord component payload culture-independent; displayed currency formatting is unchanged.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.