v0.43.0 — venice_vision: the agent can see its images (#60)

venice_vision — image understanding for the agent (#60, epic #59)

The agent could generate images but never see them — kimi-k3's #1 gap: no way to verify watermarks, character consistency across generation stages, or anatomical glitches without a human in the loop. This release closes it.

New free tool venice_vision (in venice chat --tools and venice code):

• Pass a local input_path (sent as a base64 data: URL, 25 MB cap) or an image_url; optional prompt directs the question (default: "Describe this image in detail."), plus optional model / max_tokens.
• Rides the [openai] extra through a multimodal /chat/completions call — no new dependency, not spend-gated.
• Vision-capability guard: an explicit model that advertises supportsVision: false is rejected client-side with a message naming the fix; unknown capabilities proceed. When model is omitted, the default-trait text model is used if vision-capable, else the first supportsVision model in the catalog.
• New _models.supports_capability generalizes the capability reader (supports_function_calling now delegates to it).
• Not exposed in venice mcp-serve (same as venice_models / venice_model_details).

Tests: 576 green (10 new TestVisionTool cases + tri-state capability-reader coverage).