test(search): pin currently-unasserted behaviours (Task 10 Add list)

Adds dedicated result-level tests, each indexing real documents and
asserting on matched-ID sets rather than parse shape:

- Deferred `-term` negation (G1): bare `-taxes` requires the term, and
  fielded `-title:alpha` drops the negation entirely, both matching v2.
- Reversed date ranges: absolute bounds swap (whoosh parity); relative
  `now±` bounds day-bump instead, an inconsistency pinned as a
  whoosh-compat follow-up rather than "fixed" locally.
- The six whoosh unit abbreviations (yrs/mos/wks/hrs/mins/secs).
- Unterminated `[` date range brackets 400 at the API level.
- `tag:foo,bar` comma value lists are conjunctive, with a decoy proving
  `correspondent:foo,bar` is not treated as a list.
- Date keyword phrases (`today`) honour the active (non-UTC) timezone,
  not just relative ranges.
- `_DEFAULT_SEARCH_FIELDS` stays a subset of PUBLIC_FIELDS.
- Every declared JSON subpath is actually written at index time.

Also replaces test_schema.py::TestFastFlagAgreement's tantivy-py
__reduce__() pickling probe with one built on the paperless-owned,
tantivy-independent field_descriptors().

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/documents/tests/search/test_json_subpath_completeness.py

@@ -0,0 +1,83 @@
+"""Every declared JSON subpath must actually be written to the index.
+
+PUBLIC_FIELDS declares each JSON field's subpaths (e.g. ``notes`` ->
+{"user", "note"}), but nothing coupled that declaration to what
+``_backend.py``'s document builder actually writes into the JSON blob at
+index time. A subpath declared but never written would be
+queryable-but-always-empty -- syntactically valid, silently matching
+nothing -- with no test failure anywhere.
+
+This indexes one real document carrying values for every JSON field
+(a Note, a CustomFieldInstance) and inspects the document's own stored
+JSON payload, rather than running field-specific queries: that way a
+future JSON field's subpaths are covered automatically, without a new
+per-subpath query having to be added by hand each time.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+import tantivy
+from django.contrib.auth.models import User
+from whoosh_compat import FieldKind
+
+from documents.models import CustomField
+from documents.models import CustomFieldInstance
+from documents.models import Document
+from documents.models import Note
+from documents.search._fields import PUBLIC_FIELDS
+
+if TYPE_CHECKING:
+    from documents.search._backend import TantivyBackend
+
+pytestmark = [pytest.mark.search, pytest.mark.django_db]
+
+
+class TestJsonSubpathsAreWrittenAtIndexTime:
+    def test_every_declared_json_subpath_appears_in_the_stored_document(
+        self,
+        backend: TantivyBackend,
+    ) -> None:
+        user = User.objects.create_user(username="completeness-user")
+        field = CustomField.objects.create(
+            name="Completeness Field",
+            data_type=CustomField.FieldDataType.STRING,
+        )
+        doc = Document.objects.create(
+            title="Completeness doc",
+            content="x",
+            checksum="json-subpath-completeness",
+        )
+        Note.objects.create(document=doc, user=user, note="a note")
+        CustomFieldInstance.objects.create(
+            document=doc,
+            field=field,
+            value_text="a value",
+        )
+        backend.add_or_update(doc)
+
+        index = backend._index
+        searcher = index.searcher()
+        hits = searcher.search(
+            tantivy.Query.term_query(index.schema, "id", doc.pk),
+            limit=1,
+        ).hits
+        assert hits, "the document was not indexed"
+        stored = searcher.doc(hits[0][1]).to_dict()
+
+        json_fields = [f for f in PUBLIC_FIELDS if f.kind is FieldKind.JSON]
+        assert json_fields, "no JSON fields declared - fixture is stale"
+        for field_spec in json_fields:
+            stored_values = stored.get(field_spec.name)
+            assert stored_values, (
+                f"{field_spec.name} was not written to the index at all"
+            )
+            written_keys = stored_values[0].keys()
+            for subpath in field_spec.subpaths:
+                assert subpath in written_keys, (
+                    f"{field_spec.name}.{subpath} is declared in PUBLIC_FIELDS "
+                    "but _backend.py's document builder never writes it - it "
+                    "would be queryable but always empty"
+                )