The _generate_summary() method assumed message content is always a
string (msg.get('content') or ''). When content is a multimodal list
(e.g. [{type: 'text', text: '...'}, {type: 'image_url', ...}]), this
produced mangled output: len() returned the list length instead of
character count, and slicing produced list items instead of substrings.
Add _content_to_text() helper that safely converts any content format
to plain text:
- str → returned as-is
- None → empty string
- list (multimodal) → text parts joined, images replaced with [image]
- dict/other → JSON serialization with str() fallback
This ensures multimodal conversations compress correctly instead of
producing garbled summaries.
Inspired by PR #776 by @kshitijk4poor.
1b8a1c7d5e