cleanup(sqlalchemy): pin dialect attributes and description

- ISSUE-89: pin dialect_description as "dqlite+dqlitedbapi" so
  SQLAlchemy ORM error-message formatting cannot silently change the
  rendered dialect identity across upgrades.
- ISSUE-90: declare `supports_sane_isolation_level = True` explicitly.
  dqlite always enforces SERIALIZABLE; the reported level is
  trustworthy across transactions, so SQLAlchemy skips defensive
  isolation-level resets.
- ISSUE-94: declare `supports_native_decimal = False` explicitly.
  Matches the inherited pysqlite default but pins the contract against
  upstream drift and matches the already-explicit
  `supports_native_boolean = True` pattern.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/sqlalchemydqlite/base.py

@@ -83,6 +83,16 @@ class DqliteDialect(SQLiteDialect):
     # applications introspect via ``engine.dialect.supported_isolation_levels``.
     supported_isolation_levels: tuple[str, ...] = ("SERIALIZABLE",)
 
+    # Since isolation is always SERIALIZABLE and cannot be weakened, the
+    # reported isolation level is trustworthy across transactions. SQLAlchemy
+    # skips defensive isolation-level resets when this is True.
+    supports_sane_isolation_level = True
+
+    # dqlite/SQLite have no native DECIMAL type — values are stored as TEXT
+    # or REAL. Declare explicitly to document the contract (matches pysqlite
+    # inherited default, pinned here against upstream drift).
+    supports_native_decimal = False
+
     # Override the SQLite dialect's string-based DATE/DATETIME processors:
     # dqlitedbapi returns datetime objects (PEP 249), not ISO strings.
     colspecs = {

tests/test_dialect.py

@@ -44,6 +44,21 @@ def test_create_connect_args_custom(self) -> None:
         assert kwargs["address"] == "node1:9002"
         assert kwargs["database"] == "mydb"
 
+    def test_supports_sane_isolation_level(self) -> None:
+        # dqlite always enforces SERIALIZABLE; the reported level is
+        # trustworthy across transactions (ISSUE-90).
+        assert DqliteDialect.supports_sane_isolation_level is True
+
+    def test_supports_native_decimal_false(self) -> None:
+        # SQLite/dqlite has no native DECIMAL type (ISSUE-94).
+        assert DqliteDialect.supports_native_decimal is False
+
+    def test_dialect_description(self) -> None:
+        # Pin the derived dialect_description so SQLAlchemy upgrades cannot
+        # silently change the rendered identity in ORM error messages
+        # (ISSUE-89).
+        assert DqliteDialect().dialect_description == "dqlite+dqlitedbapi"
+
 
 class TestDqliteDialectAio:
     def test_dialect_name(self) -> None: