## Summary
- Add custom `base_url` support to the Google Go model driver.
- Preserve Google URL suffix configuration when creating custom base URL
driver instances.
- Validate Google chat/stream request inputs before constructing the SDK
client.
- Cover Google model listing, connection checks, base URL resolution,
and request validation with focused tests.
## What changed
- `GoogleModel.NewInstance` now returns a Google driver configured with
the supplied base URL map.
- Google SDK client creation now resolves configured base URLs through
`genai.HTTPOptions.BaseURL`.
- Base URL lookup supports configured regions, empty-region keys, and
`default` fallback.
- Google chat, streaming chat, embeddings, and model listing now reject
blank API keys before creating SDK clients.
- Google chat and streaming chat now reject blank model names locally,
and streaming chat rejects a nil sender.
- Existing message handling, embeddings, pagination, and provider errors
are preserved.
## Why
Google custom model instances could not use configured base URLs because
`NewInstance` returned `nil` and the SDK client path ignored the driver
base URL map. The request validation keeps invalid Google calls from
reaching SDK client construction with blank credentials or incomplete
chat inputs.
### What problem does this PR solve?
Closes#14703
`GoogleModel.CheckConnection` currently returns a hardcoded `no such
method` error even though the Google Go driver already supports
`ListModels`. This makes provider connection checks fail regardless of
whether the configured API key can list Google models.
This PR makes `CheckConnection` call `ListModels`, adds a small API-key
guard for nil, empty, and whitespace-only keys, and keeps `ListModels`
useful by following paginated Google model responses.
### What stays unchanged
* Google model listing still uses the Google GenAI SDK with
`genai.BackendGeminiAPI`.
* Model names still come from `models.Items[*].Name`.
* `Balance`, `Encode`, chat, streaming, provider config, and factory
wiring are unchanged.
### Tests and validation
Added focused unit coverage for:
* `CheckConnection` delegating to `ListModels` and returning its error
* nil, missing, empty, and whitespace-only API key validation
* model-name passthrough from the list-models adapter
* paginated model listing, empty-result preservation, and next-page
error propagation
Validated current PR head `17ceef43515ba8c46c254dd349b9085bf26dcbea`
locally with Go 1.25.0:
* `go test ./internal/entity/models -run
'TestGoogleModel|TestCollectGoogleModelNames' -count=1 -v` - PASS
* `go test ./internal/entity/models -count=1` - PASS
* `go test -race ./internal/entity/models -count=1` - PASS
* `gofmt -w internal/entity/models/google.go
internal/entity/models/google_test.go` - PASS, no diff
* `git diff --check` - PASS
### Type of change
* [x] Bug Fix (non-breaking change which fixes an issue)
Co-authored-by: Jin Hai <haijin.chn@gmail.com>
