Copilot Chat OTel trace User messages are empty and system message is missing

[vritant24]

Description

While validating the fix for #299932, I noticed a new issue in the Copilot Chat OpenTelemetry traces: the system message is no longer shown, but the user messages are also empty.

In the Aspire dashboard, the trace shows multiple User input messages, but each one displays:

Steps to Reproduce

1. Enable OpenTelemetry for VS Code / Copilot Chat and point it at the standalone Aspire dashboard.
2. Use Copilot Chat with a model such as gpt-5.5.
3. Perform an operation that sends user messages and tool calls.
4. Open the generated LLM trace in Aspire.
5. View the Input & output tab.

Notes

This was observed while testing the behavior fixed for:

• Copilot chat OTel includes system instruction in message JSON #299932