Address image harness review findings

[cbusillo]

Summary

• address Claude/Gemini post-merge review findings from the no-token image replay harness
• make fake Responses request recording and response assignment atomic
• keep parsed command metadata for summaries while preserving readable scenario command strings
• scope image replay assertions to request input and require the 65536 bytes placeholder detail

Validation

• python3 -m py_compile tools/code-exec-harness/harness.py
• python3 -m json.tool tools/code-exec-harness/scenarios/image-history-replay.json >/dev/null
• python3 tools/code-exec-harness/harness.py tools/code-exec-harness/scenarios/image-history-replay.json --code-bin /Users/cbusillo/.local/bin/code --sandbox danger-full-access
• ./build-fast.sh

Refs #68
Refs #63