Summary

Fixes #30252.

HonchoMemoryProvider.sync_turn previously passed user_content /
assistant_content straight into sanitize_context, which expects a
string. When the gateway forwarded an OpenAI vision-style multimodal
turn (content as a list of {type: text|image_url, ...} parts),
sanitize_context raised expected string or bytes-like object, got 'list'.
The exception was caught and logged at WARNING by the memory manager, so
the chat completion still succeeded — but the turn was silently dropped
from Honcho's memory and the user representation never learned about
anything visual the assistant was shown.

Fix

Add a small _flatten_content helper on HonchoMemoryProvider and call
it from sync_turn before sanitisation:

• text / input_text / output_text parts contribute their text
  (non-string values coerced via str() so a malformed part can't crash
  "\n".join(...)).
• image_url / input_image parts become a literal [image] marker so
  the Honcho turn still records that visuals were exchanged.
• Other typed parts become [<type>]. Bare strings inside the list are
  kept; non-dict / non-str items are skipped.
• Plain string callers are untouched — the helper returns its input
  unchanged when it isn't a list.

Scope

Per the issue's scope note, only the Honcho plugin is patched. Other
memory providers (mem0, supermemory, byterover, retaindb,
holographic, openviking) may share the same defect at their
sync_turn(user_content: str, ...) boundary but were not investigated
in this change.

Tests

New file tests/honcho_plugin/test_sync_turn_multimodal.py — 16 tests:

• _flatten_content unit coverage: string passthrough, None,
  text+image_url flattening, input_text / output_text
  canonicalisation, non-string text coercion, input_image alias,
  unknown-type placeholder, empty list, bare strings inside lists,
  items missing type, non-dict/non-str items.
• sync_turn integration: with a mocked _manager / session, passing
  list-shaped multimodal user_content no longer raises and the
  flattened text ("what colour is this?\n[image]") reaches the
  session; list-shaped assistant_content also works; plain string
  callers still produce the unchanged messages.

$ pytest tests/honcho_plugin/test_sync_turn_multimodal.py -q
................                                                         [100%]
16 passed in 0.09s

Checklist

• Bugfix scoped to one plugin file + one new test file.
• No behavior change for string callers.
• New tests pass locally against the project venv.
• Commit SSH-signed.