feat: add auth passthrough to all namespace methods

[Kamilbenkirane]

Summary

• Add auth: Authentication | None = None parameter to every create_client() call site across all domain namespaces
• Covers all 4 domains (text, images, audio, videos) x all variants (async, sync, stream, sync.stream)

Why

The auth parameter was only passed through in TextNamespace.generate(). All other namespace methods (embed, analyze, speak, edit, generate for other domains, and all sync/stream variants) silently dropped it, making Vertex AI authentication impossible outside of async text generation.

Test plan

• uv run pytest tests/unit_tests -v

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.