### What problem does this PR solve?
Fixes custom `base_url` resolution when a model instance has no
configured region.
Some drivers read custom base URLs from `BaseURL[""]` when
`apiConfig.Region` is empty, while others normalize empty region to
`"default"` and read `BaseURL["default"]`. This PR adds the `"default"`
alias only for empty-region custom base URLs while preserving the
existing empty-region key.
Closes#15042
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Closes#15199.
The add-custom-model endpoint is routed through
`/api/v1/providers/:provider_name/instances/:instance_name/models`, but
the handler previously trusted `provider_name` and `instance_name` from
the JSON body instead of the path target. A request could therefore hit
one provider/instance URL while operating on a different body
provider/instance.
The same handler only rejected `model_types` when the slice was nil. An
empty array passed validation and reached
`ModelProviderService.AddCustomModel`, where `request.ModelTypes[0]`
could panic.
This PR makes the path provider/instance authoritative, rejects
mismatched body values, rejects missing or empty `model_types`, and adds
a service-level guard so direct service callers cannot hit the same
panic path.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### 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`).
