fix(translator): shape Anthropic non-stream responses from forced-streaming providers

The non-streaming Anthropic path leaked OpenAI chat.completion bodies to
Claude clients when the upstream provider forces streaming (forced
SSE->JSON path): parseSSEToOpenAIResponse yields choices[] which Claude
Code cannot parse. Convert to Claude Messages shape for sourceFormat
CLAUDE in sseToJsonHandler.

Shared toClaudeMessageShape/openAICompletionToClaudeMessage moved to
translator/concerns/claudeShape.js to avoid circular import between
nonStreamingHandler and sseToJsonHandler. 5 new tests: sse-to-json
reassembly + Claude guard, plus existing anthropic-nonstream-shape
updated to import from the concern.

tests/unit/anthropic-nonstream-shape.test.js

@@ -1,7 +1,7 @@
 import { describe, expect, it } from "vitest";
 
 const { stripEosSentinel } = await import("../../open-sse/translator/concerns/eosStrip.js");
-const { toClaudeMessageShape } = await import("../../open-sse/handlers/chatCore/nonStreamingHandler.js");
+const { toClaudeMessageShape } = await import("../../open-sse/translator/concerns/claudeShape.js");
 
 // An OpenAI-shape body as returned by a claude-transport executor (opencode/
 // big-pickle): no type:"message", choices[] present. This is what an Anthropic