## Problem
`TestXiaomiNewModelWithCustomDefaultTransport` panics on Go 1.25:
```
panic: interface conversion: http.RoundTripper is models.roundTripperFunc, not *http.Transport
```
In Go 1.25, `http.DefaultTransport` is no longer `*http.Transport`, so
the unchecked type assertion in `NewXiaomiModel` panics when the test
replaces it with a `roundTripperFunc`.
## Fix
Use a safe type assertion with fallback to a new `http.Transport`,
matching the pattern already used in `modelscope.go`.
## Verification
```bash
go test -run TestXiaomiNewModelWithCustomDefaultTransport ./internal/entity/models/...
# PASS
```
Internal contributors only.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>