refactor: let Artifact validator handle base64 decoding across all providers

[Kamilbenkirane]

Summary

Multiple provider clients manually call base64.b64decode() before passing data to ImageArtifact(data=...) or VideoArtifact(data=...). The Artifact base class already has a @field_validator("data", mode="before") that automatically decodes base64 strings to bytes. We should pass the base64 string directly and let the validator handle it.

Already fixed in #135 (Vertex PR) for Google providers:

• modalities/images/providers/google/gemini.py
• modalities/images/providers/google/imagen.py
• modalities/videos/providers/google/client.py

Remaining instances

• modalities/images/providers/openai/client.py (lines 64, 146)
• modalities/images/providers/xai/client.py (line 87)
• modalities/images/providers/ollama/client.py (lines 56, 118)
• modalities/images/providers/byteplus/client.py (lines 83, 165)
• modalities/videos/providers/openai/client.py (line 58)
• modalities/audio/providers/google/client.py (line 65)

Pattern

Replace:

image_bytes = base64.b64decode(base64_data)
artifacts.append(ImageArtifact(data=image_bytes, ...))

With:

artifacts.append(ImageArtifact(data=base64_data, ...))

Remove unused import base64 where applicable.

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