.NET: Dotnet: reset $LASTEXITCODE per command in persistent PowerShell sessions

[westey (westey-m)]

Motivation & Context

Dotnet equivalent of #8206

Description & Review Guide

• What are the major changes?
• What is the impact of these changes?
• What do you want reviewers to focus on?

Related Issue

Fixes #

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

🟡 Changes recommended

Consecutive native commands returning the same nonzero code cause the second command to report success.

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

Pull request overview

Updates persistent PowerShell exit-code handling to avoid stale results across commands.

Changes:

• Tracks prior $LASTEXITCODE and captures $? before flushing.
• Adds a Windows regression test for cmdlet-only commands.

File summaries

File	Description
ShellSession.cs	Revises PowerShell exit-code detection.
LocalShellExecutorTests.cs	Adds persistent-session coverage.

Review details

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

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[github-actions]

MAF Automated Review — Iteration 1

Result: Findings reported
Scope: full PR (1 commit(s)): 8cf7359c7cb3
Model: gpt-5.6-sol-fast

Overview

The PR snapshots $LASTEXITCODE, captures $? before flushing, and adds a regression test that preserves stale-code readback while preventing a later cmdlet from inheriting it. Command serialization, PowerShell-only branching, exception handling, and sentinel parsing constrain the change well. However, the value-comparison heuristic cannot distinguish stale state from a new native command returning the same nonzero code, so repeated failures can be reported as success.

Reviewed the supplied pull-request change set across correctness, security/reliability, architecture, and failure behavior.
1 verified finding remained after source verification (1 high) across 1 file. Details are attached to the affected lines below.

Affected areas: dotnet/src/Microsoft.Agents.AI.Tools.Shell/ShellSession.cs