examples/vertexai/imagegenerator: save_image_locally returns a nil error when image data is missing

## Required Information

**Describe the Bug:**

The `save_image_locally` tool detects when a loaded artifact does not contain usable inline image data, but returns the previous `err` value:

```go
resp, err := ctx.Artifacts().Load(ctx, filename)
if err != nil {
return saveImageResult{}, err
}

if resp.Part.InlineData == nil || len(resp.Part.InlineData.Data) == 0 {
log.Printf("Artifact '%s' has no inline data", filename)
return saveImageResult{}, err
}
```

Reaching the second condition means the artifact load succeeded, so `err` must be nil. The handler therefore returns an empty result with a nil error even though no image was written.

**Steps to Reproduce:**

1. Have the artifact service successfully load an artifact without inline image data, such as a text artifact:

```go
&artifact.LoadResponse{
Part: genai.NewPartFromText("not image data"),
}, nil
```

2. Invoke `saveImage` with that artifact's filename.
3. Observe the following log:

```text
Artifact '' has no inline data
```

4. The function returns `saveImageResult{}` with a nil error, and no output file is created.

**Expected Behavior:**

`saveImage` should return a descriptive non-nil error when the loaded artifact has no inline data or contains an empty inline-data payload.

**Observed Behavior:**

The missing-data condition returns the stale nil error from the successful artifact load. As a result, the tool invocation may be treated as successful even though no image was saved.

**Environment Details:**

- ADK Library Version: `main` at `a611472`
- OS: macOS arm64
- Go Version: `go1.26.6`

**Model Information:**

N/A. The behavior can be reproduced with a constructed artifact response without making a model request.

## Optional Information

**Regression:**

Unknown. The current implementation returns the error value from the preceding successful artifact load.

**Logs:**

```text
Artifact '' has no inline data
```

**Screenshots / Video:**

N/A.

**Additional Context:**

This is separate from #1477. That issue covers validation of image-generation API responses before saving an artifact. This issue concerns `save_image_locally` loading an existing artifact that does not contain usable image bytes.

The failure path should return a newly constructed error. Unit tests should cover both a nil `InlineData` field and an empty `InlineData.Data` payload.

**How often has this issue occurred?:**

Always when artifact loading succeeds but the loaded artifact contains no usable inline image data.

Start at save_image_locally in examples/vertexai/imagegenerator and inspect the branch handling a nil InlineData field or empty Data payload. Add unit coverage for both missing-data cases and run the relevant package tests. Done means the function returns a descriptive non-nil error and does not report success when no image is saved.

Written by the indexing model from the issue text.