Offline mode not working in LSP or ACP mode

While offline mode is working fine in Copilot CLI it is not working in ACP or LSP mode, even when all necessary env variables are set.
There seem to be hardcoded credential checks f.e. in "session/new".

Example:
$env:COPILOT_OFFLINE="true"
$env:COPILOT_PROVIDER_BASE_URL="http://localhost:10010/v1"
$env:COPILOT_PROVIDER_API_KEY="sk-dummy-key"
$env:COPILOT_MODEL="qwen36"
copilot --acp
{"jsonrpc": "2.0", "id": 2, "method": "session/new", "params": {"cwd": "C:/path/to/some/project", "mcpServers": []}}
{"jsonrpc":"2.0","id":2,"error":{"code":-32000,"message":"Authentication required"}}

Start by tracing the ACP and LSP handling for the session/new JSON-RPC request, especially its authentication checks, and compare that path with the working Copilot CLI offline mode. The example is complete when session/new works with COPILOT_OFFLINE=true and the provided local provider variables without returning Authentication required.

由索引模型根据 Issue 内容生成。