fix(query): _handle_keyword coerces non-iterable values to single-element list (#152)

[jensens]

Summary

Closes #152. Another ZCatalog-compat rough edge exposed after the b61 deploy freed up the code path.

Bug

```python

_handle_keyword

query_val = [query_val] if isinstance(query_val, str) else list(query_val)
```

This assumes the value is either a `str` or iterable. A Zope `DateTime`, Python `datetime`, plain `int`, or any other non-iterable scalar hits `list(value)` and raises `TypeError: 'DateTime' object is not iterable`.

Observed on aaf-6 prod via `plone.app.querystring.querybuilder.html_results` (Collection edit widget live-preview) where a caller routed a `DateTime` through a keyword-index criterion by mistake.

Fix

Treat only true iterables (`list`, `tuple`, `set`, `frozenset`) as collections; anything else — string, scalar, or object with `str` but no `iter` — is a single-value scalar. All values are coerced to `str()` to match the JSONB storage shape (keyword arrays always contain strings).

Why not block upstream instead

The upstream root cause is a buggy caller passing the wrong type. But the pgcatalog `TypeError` is too brittle — it surfaces as a 500 error for end users. The robust fix is to coerce silently; the upstream bug can be chased separately but should not take down renders.

Test plan

• Two new regression tests in `TestKeywordIndex`: `_NotIterable` stand-in for Zope DateTime, Python datetime directly.
• Existing keyword tests (single str, list or/and, default operator) unaffected.
• Full suite: 1444 passed / 23 skipped / 0 failed.
• After deploy: `@@querybuilder_html_results` on affected Collection no longer 500s.

🤖 Generated with Claude Code