[claude-code] Implement Anthropic MessageParam → Agent SDK prompt conversion

[edelauna]

Parent issue

#756

Depends on

#757

Context

The @anthropic-ai/claude-agent-sdk query() function accepts options.input_messages as MessageParam[] (same shape as the Anthropic SDK). This issue implements the conversion layer and validates edge cases before the core handler is built in the next issue.

Files

• src/api/providers/claude-code-messages.ts (new)
• src/api/providers/__tests__/claude-code-messages.spec.ts (new)

Work

• convertToClaudeCodeMessages(systemPrompt: string, messages: Anthropic.Messages.MessageParam[]): { system: string, messages: MessageParam[] } — pass-through for text content, strip or warn on unsupported block types
• Handle multi-turn conversations (alternating user/assistant)
• Handle image content blocks (pass base64 through; document that the CLI must support vision)
• Handle tool use and tool result blocks (pass through as-is — Agent SDK accepts native Anthropic tool format)

Tests

• Text-only single-turn round-trip
• Multi-turn conversation preserves message order
• Image content blocks are passed through
• Tool use / tool result blocks are passed through
• Empty messages array returns only system prompt

Acceptance criteria

• convertToClaudeCodeMessages is exported and fully typed
• All tests pass
• No runtime dependency on @anthropic-ai/claude-agent-sdk in this file (pure transformation)