fix(claude_code): flatten multi-turn history into single user envelope (2.0.4)

[CocoRoF]

Summary

Claude Code (CLI) Stage-6 sessions failed at the second turn with:

Error: CLI '/usr/bin/claude' exited with code 1:
Error: Expected message role 'user', got 'assistant'

User screenshot showed a VTuber session attempting tool calls — the first turn succeeded (assistant returned tool_use blocks), Geny executed the tools and sent the next iteration with [user, assistant_w_tool_use, user_w_tool_result], and the CLI rejected it.

Root cause

Geny's s06_api stage accumulates an Anthropic-style multi-turn message list across loop iterations. The previous build_stream_json_stdin envelopes each canonical message as {"type":"user","message":{"role":<original>, ...}} and writes one line per message. Claude Code 2.x's stream-json input parser strictly requires every envelope's message.role to be "user" — embedded assistant / tool roles are rejected.

Fix

build_stream_json_stdin collapses the entire canonical history into a single synthetic type:user envelope whose content is a markdown preamble + the current input:

## Conversation so far

### User
find the README

### Assistant
Let me check.
[Tool call: Read({"path": "/repo/README.md"})]

### Tool result
[Tool result] # Hello

## Current input
summarize it

The LLM reconstructs the conversation from the structure; the CLI sees one cohesive single-turn prompt.

Provider-neutrality

Per the user's directive — every provider (anthropic / openai / google / vllm / claude_code_cli / copilot_cli) must accept the same canonical message-list shape and translate internally. The CLI's stream-json input grammar is strict user-only; the executor owns the translation so hosts never see the difference. Combined with the 2.0.3 StreamJsonAccumulator (output contract), the full input + output round-trip is now provider-symmetric.

Edge cases handled

• Single-turn fast path: one user message → emit the canonical envelope unchanged (byte-for-byte legacy compat)
• Thinking blocks: dropped (CLI does its own --effort thinking on the new turn)
• Tool errors: is_error: True tool_results render under a [Tool error] tag instead of [Tool result]
• Image blocks: render as [image attachment] placeholder
• String + list content: both shapes handled

Test plan

• pytest tests/llm_client/ 189/189 pass
• New regression guards:
  • test_stdin_envelope_multi_turn_always_user_role
  • test_stdin_envelope_multi_turn_preserves_history_in_content
  • test_stdin_envelope_drops_thinking_and_handles_tool_errors
• Manual on prod: VTuber session with env=claude_code_cli, send "워커한테 메시지 보내서 자기소개 해" → multi-turn loop completes without "Expected message role 'user'" error

Release

2.0.4. Migration: none.