### What problem does this PR solve?
Adds a shared safe default implementation for unsupported Go
model-driver capability methods and migrates the confirmed panic-stub
providers to use it.
The Go `ModelDriver` interface requires providers to implement many
capability methods even when the provider does not support them. XunFei
had unsupported capability methods implemented as `panic("implement
me")`, Mistral still had a panic in `ParseFile`, and HuaweiCloud carried
an unreachable `panic("implement me")` after a normal chat return.
### Type of change
- [x] Refactoring
Co-authored-by: Haruko386 <tryeverypossible@163.com>