… estimation and secret redaction

## Problem

Two related bugs affected users sending multimodal messages (text + images):

**Bug 1 — Wrong token estimation (`_find_tail_cut_by_tokens`, line ~1080)**
When a message's `content` field is a list (multimodal format), `len(content)`
returns the number of list elements (typically 1–3), not the character count.
A message with 4000 chars of text was estimated at ~10 tokens instead of ~1000,
causing the context compression boundary to be placed far too early — retaining
much less tail context than intended for vision-heavy conversations.

**Bug 2 — Secret redaction bypassed (`_serialize_for_summary`, line ~603)**
`redact_sensitive_text(msg.get("content") or "")` relies on a non-empty list
being truthy: a list is truthy so the `or ""` fallback is skipped and
`redact_sensitive_text` receives a `list` instead of a `str`. This means any
API keys or tokens embedded in the text parts of multimodal messages are
forwarded unredacted to the auxiliary summarisation model and persisted in the
compressed summary — a silent security regression for users of vision features.

## Fix

Both fixes follow the pattern already used correctly at line 478
(`_prune_old_tool_results`):

```python
content_len = (
    sum(len(p.get("text", "")) if isinstance(p, dict) else len(str(p))
        for p in raw_content)
    if isinstance(raw_content, list)
    else len(raw_content)
)
```

For redaction, text parts are individually redacted and joined; image parts
(no `"text"` key) contribute nothing to the summary text — consistent with
the existing serialisation behaviour.

## Tests

Five new tests in `TestMultimodalContent` cover:
- Token budget is exhausted by a large multimodal text part (not the list length)
- Image-only content does not crash and costs near-zero tokens
- Secrets in multimodal text parts are redacted before summary serialisation
- Image-only content serialises to an empty contribution without error
- Plain string content regression guard

Co-Authored-By: Tianye Song <songtianye1997@gmail.com>