fix: respect USE_USER_TOKEN config in OpenAI endpoint (#2187)
* fix: respect USE_USER_TOKEN config in OpenAI endpoint
Previously, the OpenAI endpoint wrapper unconditionally injected the
user's OIDC token into the Authorization header if a session existed.
This overwrote the OPENAI_API_KEY, causing 401 Unauthorized errors
when using external providers alongside OIDC authentication.
This commit updates the header injection to properly check if
USE_USER_TOKEN is set to "true" before passing the user token,
restoring compatibility with external OpenAI-compatible backends.
* fix: respect USE_USER_TOKEN in MCP MCP flow
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Co-authored-by: Victor Muštar <victor.mustar@gmail.com>