Fix interpreting baud rate bitmask as decimal value

[mrek-msft]

Intended to fix !119105.

[mrek-msft]

@dotnet-policy-service agree company="Microsoft"

[Copilot]

Pull request overview

This pull request fixes an issue where baud rate bitmask values from Windows drivers were being incorrectly interpreted as decimal values, causing validation errors. Some device drivers (like Silicon Labs USB to UART converters) return a decimal baud rate value instead of a bitmask, while Windows documentation specifies bitmasks should be used.

Changes:

• Added a new helper method ConvertMaxBaudBitMaskToBaudRate to intelligently interpret the dwMaxBaud field as either a bitmask or decimal value
• Updated the BaudRate property setter to use the converted value for validation
• Updated the SerialStream constructor to use the converted value for validation

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[MihaZupan]

Is there a way for us to test this in CI without a weird device/driver?

[mrek-msft]

Is there a way for us to test this in CI without a weird device/driver?

It is possible to run it on virtual machine with emulated ports. Not sure if it runs in this way in CI though.