Reset server MAC after TransformFinalBlock on .NET FW

[scott-xu]

Call _serverMac.Initialize() after TransformFinalBlock when running on .NET Framework. This ensures the MAC is properly reset, addressing differences in cryptographic API behavior across different versions of mscorlib.dll.

Close #1829

[Copilot]

🟡 Changes recommended

MAC reinitialization should be unconditional per the API contract, with regression coverage for repeated verification.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Fixes receive-MAC reuse after TransformFinalBlock, addressing second-packet failures on older .NET Framework builds.

Changes:

• Resets server MAC state in ETM and non-ETM receive paths.
• Applies resets conditionally to .NET Framework.

File summaries

File	Description
src/Renci.SshNet/Session.cs	Resets receive-MAC state after packet verification.

Review details

Suppressed comments (1)

src/Renci.SshNet/Session.cs:1386

• The same API contract applies in the non-ETM path: after TransformFinalBlock, reusing the instance without Initialize() is not guaranteed on any target framework. Remove the conditional compilation here as well.

#if NETFRAMEWORK
                // Ensure MAC is reset so that it can be reused for the next packet
                // It is only required for certain older .NET Framework builds, e.g., mscorlib.dll version 4.8.4110.0
                _serverMac.Initialize();
#endif

• Files reviewed: 1/1 changed files
• Comments generated: 2
• Review effort level: Balanced

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[WojciechNagorski]

LGTM. I verified this PR using Copilot and it produced the same solution. #1834

I think it's good to leave #if NETFRAMEWORK, so that this change only applies to the old framework on which this problem occurs.