Improve CLI communication error diagnostics

[Copilot]

Users experiencing CLI startup or connection failures receive generic "JSON-RPC connection lost" errors with no actionable information about the root cause (missing CLI, authentication, configuration).

Changes

Error Detection & Reporting

• Early failure detection: 50ms check after process start captures immediate crashes with exit codes and stderr
• Synchronized stderr capture: Avoids race conditions between immediate checks and background logging
• Specific exception handling: ConnectionLostException now includes troubleshooting link and context
• Process startup errors: Wrapped Process.Start() to catch and report missing executable errors

Documentation

• Troubleshooting guide (docs/troubleshooting-connection-errors.md): Covers CLI installation, authentication, version compatibility, port conflicts, permissions
• Debug logging examples: Cross-language examples for enabling detailed diagnostics
• Getting started integration: Link from main tutorial to troubleshooting

Testing

• Platform-aware error tests: Verify detection of missing CLI and immediate process exits on Windows and Unix

Error Message Improvements

Before:

IOException: The JSON-RPC connection with the remote party was lost before the request could complete.

After:

InvalidOperationException: Copilot CLI process exited immediately with code 1.
This may indicate the CLI is not properly installed or configured.
Error output:
Error: Not authenticated. Please run 'copilot auth login' first.

IOException: Communication error with Copilot CLI: The connection was lost.
This usually means the CLI process crashed or exited unexpectedly.
See https://github.com/github/copilot-sdk/blob/main/docs/troubleshooting-connection-errors.md

Technical Notes

Stderr capture uses a synchronized list to ensure error output is available regardless of timing between immediate exit checks and background logging. The 50ms delay balances false positives (slow-starting but valid processes) against detection speed.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Communication error with Copilot CLI: The JSON-RPC connection with the remote party was lost before the request could complete.</issue_title>
<issue_description>I followed the getting started guide as mentioned here.

I created a new console app using .NET 10 and included the Github Copilot SDK(0.1.19).

When running the application, I gett the following error on the CreateSessionAsync line.

An exception of type 'System.IO.IOException' occurred in System.Private.CoreLib.dll but was not handled in user code
Communication error with Copilot CLI: The JSON-RPC connection with the remote party was lost before the request could complete.

Executing the CLI directly works without any problem.
</issue_description>

<agent_instructions>Investigate the issue and provide improvements and/or suggestions</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@patniko Would be helpful to have a repro. Mind sharing CLI and SDK version #? We'll investigate other ways this might occur in parallel.

• Fixes Communication error with Copilot CLI: The JSON-RPC connection with the remote party was lost before the request could complete. #253

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