Fix _decode_default_value to unescape doubled single quotes in string defaults (#811)

* Fix _decode_default_value to unescape doubled single quotes in string defaults

SQLite stores string defaults with single quotes doubled (e.g. DEFAULT 'O''Brien'
is stored as the literal "'O''Brien'" in sqlite_master). The previous code
stripped the outer quotes with value[1:-1] but never converted '' back to ',
so default_values returned the raw escaped form instead of the true string value.

* Test for doubled single quotes in string defaults

sqlite_utils/db.py

@@ -5108,8 +5108,8 @@ def resolve_extracts(
 
 def _decode_default_value(value: str) -> object:
     if value.startswith("'") and value.endswith("'"):
-        # It's a string
-        return value[1:-1]
+        # It's a string; unescape doubled single quotes
+        return value[1:-1].replace("''", "'")
     if value.isdigit():
         # It's an integer
         return int(value)

tests/test_introspect.py

@@ -312,6 +312,7 @@ def test_table_strict(fresh_db, create_table, expected_strict):
         1,
         1.3,
         "foo",
+        "O'Brien",
         True,
         b"binary",
     ),
@@ -324,6 +325,16 @@ def test_table_default_values(fresh_db, value):
     assert default_values == {"value": value}
 
 
+def test_table_default_values_escaped_quotes(fresh_db):
+    # SQLite stores string defaults with single quotes doubled, so
+    # introspection needs to unescape them again
+    fresh_db.execute(
+        "create table t (id integer primary key, name text default 'O''Brien')"
+    )
+    assert "default 'O''Brien'" in fresh_db["t"].schema
+    assert fresh_db["t"].default_values == {"name": "O'Brien"}
+
+
 def test_pks_use_primary_key_declaration_order(fresh_db):
     # PRIMARY KEY (a, b) declared against columns stored in order (b, a) -
     # pks must follow the declaration order, which is what SQLite uses to