Follow-up on @AlexFucuson9's cherry-picked multimodal flattening:
- http(s) image parts render as '[image: <url>]' so the summary keeps a
referenceable handle after compaction (base64 data: URLs still
collapse to '[image]' — no reusable reference, and leaking them is
the bug being fixed)
- unknown part types (document, future shapes) render as '[<type>]'
instead of being silently dropped
- test docstrings corrected: the original crash premise is stale
(redact_sensitive_text grew str() coercion in #52147); the live bug
is repr-noise + base64 leakage into the summarizer input
msg.get('content') can return a list of parts for multimodal messages
(containing text, images, etc.). The old code passed this list directly
to redact_sensitive_text(text: str), which raised AttributeError on
list.replace(), causing context compression to fail entirely for any
session with attached images.
Fix: detect list content and extract text parts before redacting.
Image parts are replaced with '[image]' placeholder.
