fix(vertexai): use correct MIME type for multimodal stream sample

[amcolin]

Description

This PR addresses two issues in the generativeaionvertexai_stream_multimodality_basic sample:

1. Corrected MIME Type: Updated it to the supported format to ensure successful multimodal streaming.
2. Error Handling: Added explicit nil-pointer checks and robust error handling. This prevents the sample from panicking if the API response or specific fields are unexpected, improving the overall stability of the snippet.

Fixes Internal b/506216658

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request corrects a MIME type for an image file and enhances the robustness of the streaming response loop by adding a nil check for the response and providing more descriptive error messages. A review comment suggests further improving the safety of the response handling by checking for a nil Content field within the first candidate to prevent potential panics.