### What problem does this PR solve?
- Implements the `Encode` method in the Google Gemini driver, which was
previously a stub returning `not implemented`
- Uses the `google.golang.org/genai` SDK's `EmbedContent` API, which
routes to the `batchEmbedContents` endpoint internally — all texts are
sent in a single request
- Adds `text-embedding-004` (max 2048 tokens) to
`conf/models/google.json`
- Response values are `[]float32` from the SDK and are cast to
`[]float64` to satisfy the `ModelDriver` interface
## Files changed
- `internal/entity/models/google.go` — full `Encode` implementation
- `conf/models/google.json` — adds `text-embedding-004` embedding model
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
As title.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
---------
Signed-off-by: Jin Hai <haijin.chn@gmail.com>
