fix: keep string leaves out of JSON_EACH in list-member filters

[arotolo]

The list-member overload, Filter(type, x => x.Items, x => x.Member, value),
walks the list with JSON_TREE and handed every node it yielded to JSON_EACH.
JSON_TREE yields the scalar leaves as well as the element objects, and a
string leaf -- any string member, or a DateTime, which both serializers write
as text -- is not JSON, so JSON_EACH raised SQLite error 1 "malformed JSON".
EXISTS stops at the first element that satisfies the predicate, so the error
only surfaced for documents whose elements never match: the filter appeared
to work until the store held a row it had to reject. The one existing test
never set a string member, so it never reached a leaf.

JSON_EACH now receives its input through a CASE that yields NULL for any node
that is not an object, and JSON_EACH on NULL produces no rows. This is safe by
SQL semantics rather than by planner placement: a JT.type conjunct in the
WHERE clause is also coded in the outer loop today, but the CASE form does not
depend on that. Arrays are excluded along with scalars, since a $.Member path
cannot resolve on one, and JSON_TREE still visits nested lists of objects, so
the depth searched is unchanged.

Writing the NotEquals test surfaced a second defect on the same path: a
DateTime value was rendered with ToString(), "1/1/2026 8:00:00 AM" under
en-US, against a stored "2026-01-01T08:00:00Z", so no element ever compared
equal and every document with a non-empty list matched. NotEquals now
formats through DateTimeSerializationFormat as Equals already did. The
top-level NotEquals branch has the same defect and is left for a separate
change.

The shape dates from the original inner-list commit and first shipped in
v2.0.0.

Tests: 318 passed, 4 pre-existing skips; 8 new.

[Copilot]

Copilot review overview

🟢 Approval recommended

All reviewed changes are covered by tests and no blocking issues remain.

Review effort: Lite
Findings: None

What changed in this PR

Fixes list-member filters that could fail on scalar JSON leaves and corrects DateTime comparisons for NotEquals.

Changes:

• Restricts JSON_EACH inputs to object nodes.
• Uses serializer-formatted DateTime values.
• Adds regression tests and changelog entries.

File	Description
TychoDB/​FilterBuilder.cs	Fixes JSON traversal and DateTime inequality formatting.
TychoDB.UnitTests/​ListMemberFilterTests.cs	Adds list-member filter regression coverage.
CHANGELOG.md	Documents both fixes.

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