### 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 Replicate as a chat provider backed by the documented predictions
API
- Register Replicate in the Go model factory and provider config
- Support non-streaming chat through sync predictions, polling fallback,
streaming through `urls.stream`, model listing, and connection checks
## Notes
- Uses `POST /v1/predictions` with Replicate model identifiers in
`version`, which supports official and community model identifiers
- Maps RAGFlow messages into Replicate prompt-shaped inputs (`prompt`,
optional `system_prompt`) and forwards common documented LLM inputs:
`max_new_tokens`, `temperature`, `top_p`
- Preserves whitespace in SSE output chunks and emits RAGFlow `[DONE]`
at stream completion
## Tests
- `go test -vet=off -run TestReplicate -count=1
./internal/entity/models`
- `go test -vet=off -count=1 ./internal/entity/models`
Refs #14736
