vision: Claude provider structured content with image blocks

[bug-ops]

Parent: #490
Depends on: #491

Scope

Implement vision support in the Claude/Anthropic provider.

Changes

1. crates/zeph-llm/src/claude.rs
   • Override supports_vision() -> true
   • Add AnthropicContentBlock::Image { source: ImageSource } variant
   • ImageSource: { type: "base64", media_type: String, data: String }
   • In split_messages() / request builder: when MessagePart::Image is present, switch from plain string content to StructuredContent::Blocks format containing both text and image blocks

Notes

• Claude API expects base64-encoded images in structured content blocks
• Supported formats: JPEG, PNG, GIF, WebP
• Max image size: 20MB per the API docs