### What problem does this PR solve?
Closes#15089.
Adds PPIO support to the Go model-provider layer so PPIO instances can
be routed through the Go API server with the same OpenAI-compatible
chat, streaming, model listing, and connection-check flow used by other
SaaS providers.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
## Summary
- Added a PPIO Go model driver.
- Added the PPIO provider catalog and default OpenAI-compatible API URL.
- Registered PPIO in the model factory.
- Added focused provider and provider-manager tests.
## What changed
- Implemented chat completions, SSE streaming, ListModels, and
CheckConnection for PPIO.
- Covered request shape, stream termination, reasoning fallback, model
listing, custom base URLs, safe transport setup, unsupported methods,
and provider config loading.
- Kept the provider catalog aligned with the existing RAGFlow PPIO
factory model set.
- Cleaned up pre-existing Go model package validation blockers so the
scoped provider tests can run normally with vet enabled.
## Why
The existing Python/provider catalog path includes PPIO, but the Go
model-provider layer did not have a PPIO driver, so the Go API server
could not instantiate or use PPIO as requested in #15089.
## What
- Add TogetherAI as a chat provider backed by its OpenAI-compatible
`/v1/chat/completions` API
- Register TogetherAI in the Go model factory and provider config
- Support non-streaming chat, SSE streaming chat, model listing, and
connection checks
## Notes
- Uses the current TogetherAI OpenAI-compatible base URL
`https://api.together.ai/v1`
- Forwards documented chat parameters from `ChatConfig`: `max_tokens`,
`temperature`, `top_p`, `stop`, and GPT-OSS `reasoning_effort`
- Routes Together reasoning traces from `reasoning` /
`reasoning_content` into `ReasonContent`
## Tests
- `go test -vet=off -run TestTogetherAI -count=1
./internal/entity/models`
- `go test -vet=off -count=1 ./internal/entity/models`
Refs #14736
