fix(proxy): preserve mixed user content in Responses

[LPFchan]

Summary

• preserve text and image blocks that accompany tool_result blocks in Anthropic user messages
• emit Responses input items in source order around each function_call_output
• add regression coverage for mixed text, images, multiple tool results, and ordering

Root cause

The Responses translator selected an all-or-nothing tool_result path whenever any block in a user message was a tool result. That path emitted the tool outputs and silently discarded every sibling block. Claude Code may combine a tool result with subsequent user guidance to preserve role alternation, so corrective prompts could disappear while the model continued the previous task.

Closes #8.

Verification

• cargo fmt --check
• cargo test (345 unit tests and 1 integration test)
• cargo clippy -- -D warnings
• git diff --check