fix(examples): return an error for missing image data

[ktsoator]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: examples/vertexai/imagegenerator: save_image_locally returns a nil error when image data is missing #1508

Problem:

save_image_locally detected artifacts without usable inline data but returned
the stale error value from the preceding successful artifact load. Because that
error was nil, the tool invocation could be treated as successful even though
no image was written.

Solution:

Return a descriptive error when the loaded artifact has no InlineData or its
inline payload is empty.

Add tests using the in-memory artifact service for both missing-data shapes and
verify that artifact load errors continue to be returned unchanged.

Behavior change

Calling save_image_locally with an artifact that has no usable inline data now
returns a non-nil error instead of an empty result with a nil error.

Testing Plan

Unit Tests:

• All unit tests pass locally.

With the source change reverted, TestSaveImageRejectsMissingInlineData fails
for both nil inline data and empty inline data because saveImage returns a
nil error.

Commands completed successfully:

• go test -race -count=1 ./examples/vertexai/imagegenerator
• go build -mod=readonly work
• go test -race -mod=readonly -count=1 -shuffle=on work
• go vet ./examples/...
• golangci-lint run with the CI-pinned v2.3.1 in both modules
• go mod tidy -diff in both modules
• golangci-lint fmt -d with the CI-pinned v2.3.1 in both modules

Manual End-to-End (E2E) Tests:

Not run. This failure path does not require a live model or API request and is
covered offline using the real in-memory artifact service.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code where necessary.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

This is separate from #1477, which covers validation of image-generation API
responses before saving an artifact. This change concerns loading an existing
artifact in save_image_locally.

[wolo-lab]

LGTM, thanks