fixes PSP commandline encoding

[LowLevelMahn]

Description of Changes

change the encode/decoding of the PSP Commandline

Rationale behind Changes

Behavior was different to native DOS or dosbox, did not work properbly

Suggested Testing Steps

there is a DosProgramSegmentPrefixCmdTest to test encoding/decoding with some cases

[Copilot]

Pull request overview

This PR fixes the PSP (Program Segment Prefix) command line encoding/decoding to match native DOS and DOSBox behavior. The implementation addresses incorrect handling of spaces, length calculation, and termination characters in the DOS command tail structure.

Key changes:

• Introduces PrepareCommandlineString method to normalize command line arguments with proper DOS formatting (leading space, trailing whitespace trimming, length limits)
• Refactors DosCommandTail.Command property with explicit ASCII encoding/decoding instead of relying on zero-terminated string helpers
• Adds comprehensive test coverage with multiple command line scenarios including empty strings, whitespace handling, and special characters

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 9 comments.

File	Description
tests/Spice86.Tests/Dos/DosProgramSegmentPrefixCmdTest.cs	New test file with 6 test cases validating command line encoding against DOS byte format expectations
src/Spice86.Core/Emulator/OperatingSystem/Structures/DosCommandTail.cs	Adds PrepareCommandlineString and CheckParameterString methods; refactors Command getter/setter with explicit byte-level encoding; updates constants
src/Spice86.Core/Emulator/OperatingSystem/DosProcessManager.cs	Removes obsolete ArgumentsToDosBytes method; simplifies PSP command tail setup to use new PrepareCommandlineString

[maximilien-noal]

Approved with a few suggestions. :)

Thank you for this @LowLevelMahn !

Really appreciate it.

It's much better than the old code.

[maximilien-noal]

could be ag = $" {ag}";

[maximilien-noal]

To be visible for users of the Nuget package, this comment could be a ///

above the const.

[kevinferrare]

Wouldnt this work?

        get => Encoding.ASCII.GetString(GetUInt8Array(1, Length).ToArray());