### What problem does this PR solve?
Closes#15029.
Some custom `base_url` paths in `ModelProviderService` call
`NewInstance(newURL)` and then immediately invoke methods on the
returned driver. Several real Go model drivers still return `nil` from
`NewInstance`, so those paths can panic instead of returning a normal
error.
This PR:
- centralizes custom base URL driver creation in `model_service.go`
- skips request-local driver creation when `base_url` is blank or
whitespace
- preserves the existing region key behavior when building the
request-local base URL map
- returns a clear error when the provider driver is missing or
`NewInstance` returns `nil`
- routes list/check/task and active model paths through the guarded
helper
- adds focused unit coverage for empty-region preservation, regional
base URLs, blank base URLs, nil drivers, and nil `NewInstance` results
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### Test plan
- [x] `git diff --check upstream/main...HEAD`
- [x] `/root/go/bin/gofmt -w internal/service/model_service.go
internal/service/model_service_test.go`
- [x] `GOPATH=/root/gopath GOTOOLCHAIN=local /root/go/bin/go test
./internal/service -run TestNewModelDriverForBaseURL -count=1 -vet=off`
- [x] `GOPATH=/root/gopath GOTOOLCHAIN=local /root/go/bin/go build
./internal/service/... ./internal/entity/models/...`
Note: the same targeted `go test` command without `-vet=off` is
currently blocked by an existing unrelated vet finding in
`internal/service/llm.go:355` (`non-constant format string in call to
fmt.Errorf`).
