fix(agent): skip persist_user_message override when content is a multimodal list

[liuhao1024]

What does this PR do?

Fixes _apply_persist_user_message_override() clobbering multimodal user message content (image blocks, audio blocks) with a plain text string, causing vision models to never receive attached images sent via the ACP adapter.

Related Issue

Fixes #44242

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• run_agent.py: Added isinstance(msg.get("content"), list) guard in _apply_persist_user_message_override() — when the user message content is a list (multimodal: text + image_url, etc.), skip the override entirely. Text-only string content is still overridden as before.
• tests/run_agent/test_run_agent.py: Added test_persist_session_preserves_multimodal_content to verify multimodal content lists are preserved and not clobbered by the persist override.

How to Test

1. Configure a vision-capable model (e.g. openrouter/google/gemini-2.5-flash-lite)
2. Send an image via ACP session/prompt with a multimodal content block
3. Before fix: model replies "IMAGE_NOT_RECEIVED" — image was dropped
4. After fix: model correctly identifies the image content
5. Run pytest tests/run_agent/test_run_agent.py::TestPersistUserMessageOverride -v — both tests pass

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: run_agent.py::_apply_persist_user_message_override (callers: 2 in _persist_session and run_conversation)
• Blast radius: LOW — guard only adds an early-return for list content; text-only paths unchanged
• Related patterns: ACP adapter passes persist_user_message as plain string; _persist_session and run_conversation both call the override

[draix]

Reviewed the fix and both competing PRs (#44247 and #44248 — the latter is being closed as a duplicate per the collaborator note). Here's my analysis:

Fix correctness: ✅ Correct and minimal

The one-liner guard is the right approach:

if isinstance(msg.get("content"), list):
    return

_apply_persist_user_message_override was designed for synthetic-prefix cleanup on text turns (voice prefix in cli.py, ACP steering text). Multimodal content lists have never been an intended target of this override, so an unconditional early-return for list content is safe and correct. The blast radius is LOW — only the list-content path changes; all existing text-turn behavior is unchanged.

DB persistence: ✅ Still covered

Image redaction for the session DB ([screenshot] placeholder) already runs downstream in _flush_messages_to_session_db, so skipping the override here doesn't cause raw base64 image data to land in the session DB.

One suggestion worth considering from #44248:

The sibling PR's test (test_override_leaves_multimodal_content_intact) calls _persist_session rather than _apply_persist_user_message_override directly, and also asserts on the DB write:

first_db_write = agent._session_db.append_message.call_args_list[0].kwargs
assert first_db_write["content"] == "What color is this?\n[screenshot]"

This gives you a full end-to-end regression for the actual failure scenario (the crash-resilience persist path) and ensures the DB redaction path isn't accidentally broken. Consider adding that assertion — or a similar one — to this test for belt-and-suspenders coverage.

Otherwise LGTM. The fix is correct, the existing text-turn test is untouched, and the approach matches the description in the original issue.

[teknium1]

Closed in favor of #47907 (merged). You submitted the same one-line fix first (by ~2.5 min) — credited in the salvage PR. @Rivuza's #44249 was merged because it carried a comment + an extra positive-path regression test, but both of you nailed the same root cause. Thanks for catching this! Closes #44242.