Chore: make the backend test suite pass on MariaDB

Make the reference predicates for the string custom field filters follow
the column collation, which is case insensitive on MariaDB, and quote
identifiers in the effective content query matchers with the backend's
own quoting.

src/documents/tests/test_api_filter_by_custom_fields.py

@@ -6,6 +6,7 @@ from unittest.mock import patch
 from urllib.parse import quote
 
 import time_machine
+from django.db import connection
 from django.test import override_settings
 from django.utils import timezone
 from rest_framework.test import APITestCase
@@ -20,6 +21,13 @@ from paperless_testing.dirs import DirectoriesMixin
 from paperless_testing.factories import UserFactory
 
 
+def _exact_equal(value: str | None, expected: str) -> bool:
+    """`exact` follows the column collation, which is case insensitive on MariaDB."""
+    if connection.vendor == "mysql" and value is not None:
+        return value.casefold() == expected.casefold()
+    return value == expected
+
+
 class DocumentWrapper:
     """
     Allows Pythonic access to the custom fields associated with the wrapped document.
@@ -287,7 +295,8 @@ class TestCustomFieldsSearch(DirectoriesMixin, APITestCase):
         self._assert_query_match_predicate(
             ["string_field", "exact", "paperless"],
             lambda document: (
-                "string_field" in document and document["string_field"] == "paperless"
+                "string_field" in document
+                and _exact_equal(document["string_field"], "paperless")
             ),
         )
 
@@ -296,7 +305,10 @@ class TestCustomFieldsSearch(DirectoriesMixin, APITestCase):
             ["string_field", "in", ["paperless", "Paperless"]],
             lambda document: (
                 "string_field" in document
-                and document["string_field"] in ("paperless", "Paperless")
+                and any(
+                    _exact_equal(document["string_field"], expected)
+                    for expected in ("paperless", "Paperless")
+                )
             ),
         )
 
@@ -761,7 +773,7 @@ class TestCustomFieldsSearch(DirectoriesMixin, APITestCase):
             lambda document: (
                 not (
                     "string_field" in document
-                    and document["string_field"] == "paperless"
+                    and _exact_equal(document["string_field"], "paperless")
                 )
             ),
         )

src/documents/tests/test_document_list_effective_content_annotation.py

@@ -242,9 +242,7 @@ class TestDocumentListEffectiveContentAnnotation:
         # Only the list query itself reads a content column: no extra query
         # for the skipped prefetch, none for a per-instance fallback
         content_queries = [
-            query
-            for query in ctx.captured_queries
-            if '"documents_document"."content"' in query["sql"]
+            query for query in ctx.captured_queries if _content_column() in query["sql"]
         ]
         assert len(content_queries) == 1
 
@@ -365,6 +363,11 @@ def _get_document_queries(
     return [q for q in ctx.captured_queries if '"django_content_type"' not in q["sql"]]
 
 
+def _content_column() -> str:
+    quote = connection.ops.quote_name
+    return f"{quote('documents_document')}.{quote('content')}"
+
+
 def _get_effective_content_fallback_queries(
     ctx: CaptureQueriesContext,
 ) -> list[dict[str, str]]:
@@ -379,7 +382,7 @@ def _get_effective_content_fallback_queries(
     return [
         q
         for q in ctx.captured_queries
-        if q["sql"].startswith('SELECT "documents_document"."content" FROM')
+        if q["sql"].startswith(f"SELECT {_content_column()} FROM")
     ]