[claude-code] Implement ClaudeCodeHandler and wire into provider factory

[edelauna]

Parent issue

#756

Depends on

#757 #758

Context

Core handler class following the same BaseProvider pattern as OpenAiCodexHandler. Uses @anthropic-ai/claude-agent-sdk query() with includePartialMessages: true to stream text deltas, and maps SDKResultError to ApiProviderError.

Files

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

Work

• ClaudeCodeHandler extends BaseProvider implements SingleCompletionHandler
• createMessage(): call query() with includePartialMessages: true, iterate events:
  • SDKPartialAssistantMessage (type: "stream_event") → yield { type: "text", text } from content_block_delta events
  • SDKResultMessage (subtype: "error") → throw ApiProviderError
• getModel(): return selected model id + info from claudeCodeModels
• completePrompt(): single-turn, collect text from result
• case "claude-code" in src/api/index.ts factory switch
• Export from src/api/providers/index.ts
• Resolve CLI binary path via options.claudeCodeCliPath → which claude fallback

Tests

Mock @anthropic-ai/claude-agent-sdk:

• Text streaming yields correct { type: "text" } chunks
• SDKResultError maps to ApiProviderError
• completePrompt() returns concatenated text
• Abort signal propagates to query() options
• Factory returns ClaudeCodeHandler for "claude-code" provider

Acceptance criteria

• Handler streams text from a mocked Agent SDK
• Errors surface as ApiProviderError with provider name and model id
• Factory switch correctly instantiates the handler
• All tests pass