fix(go-models): apply custom Google base URLs (#15385)

## 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.