When proxying through another new-api instance, the upstream
X-Oneapi-Request-Id was overwriting the local one in client responses.
This adds a new `upstream_request_id` field to the logs table, captures
the upstream ID during relay, and filters it from being copied back to
the client. Frontend gains search/filter and detail display support.
- Introduced new OpenAI text models in `common/model.go`.
- Added `IsOpenAITextModel` function to check for OpenAI text models.
- Refactored token estimation methods across various channels to use estimated prompt tokens instead of direct prompt token counts.
- Updated related functions and structures to accommodate the new token estimation approach, enhancing overall token management.
