Fix transform() corrupting TRUE/FALSE/NULL column defaults into strings

[gaoflow]

The bug

table.transform() corrupts TRUE / FALSE / NULL keyword-literal column defaults into string literals, silently changing the value future rows receive.

import sqlite_utils
db = sqlite_utils.Database(memory=True)
db.execute("CREATE TABLE t (id INTEGER PRIMARY KEY, is_active INTEGER DEFAULT TRUE)")
db["t"].insert({"id": 1})
db["t"].transform(rename={"id": "pk"})          # any structure-only rebuild
print(db["t"].schema)                            # ... "is_active" INTEGER DEFAULT 'TRUE'
db["t"].insert({})
print(db.execute("SELECT is_active FROM t").fetchall())   # [(1,), ('TRUE',)]  <-- corrupted

Before the transform a default insert stored the integer 1; afterwards it stores the text 'TRUE'. Same for FALSE (→ 'FALSE' instead of 0) and NULL (→ 'NULL' instead of null). On an INTEGER column the quoted 'TRUE' isn't numeric, so it's stored as TEXT — a genuine type and value change.

Cause

transform() rebuilds the table by re-emitting each column's PRAGMA table_info default through quote_default_value(). That method returns the value verbatim when it is already quoted, is a CURRENT_TIME/DATE/TIMESTAMP literal, or ends with ); otherwise it calls self.quote(). The keyword literals TRUE, FALSE and NULL match none of those guards, so they get string-quoted.

The fix

Return TRUE / FALSE / NULL unquoted, mirroring the existing CURRENT_* handling. A populated/numeric/expression default is unaffected.

Tests

Added TRUE / FALSE / true / NULL cases to the quote_default_value EXAMPLES table, and a functional test_transform_preserves_keyword_literal_defaults that creates DEFAULT TRUE/FALSE/NULL, rebuilds via transform(), and asserts the schema stays unquoted and a fresh default insert still yields 1 / 0 / NULL. All new cases fail on main and pass with the fix; the full test_default_value.py + test_transform.py suites stay green (78 passed); black, flake8 and mypy clean.

---

📚 Documentation preview 📚: https://sqlite-utils--764.org.readthedocs.build/en/764/