Add in-process app-server parent request headers

[nicholasclark-openai]

Summary

• add bootstrap-only parent request headers to the in-process app-server startup args
• forward those headers into fresh thread creation only
• apply them to outbound Responses and MCP HTTP requests

Scope

• in-process startup args only
• no remote initialize protocol changes
• no SessionConfiguration field for parent request headers

Testing

• not run (per request)