fix(sqlalchemy-bigquery): compile func.now() to CURRENT_TIMESTAMP (#18994)

Fixes #15762

BigQuery has no `NOW()` function, but `func.now()` compiled to `now()`
with this dialect, so queries and `server_default=func.now()` columns
failed in BigQuery.

This adds `visit_now_func` to `BigQueryCompiler` so `func.now()`
compiles to `CURRENT_TIMESTAMP`. That is what `func.current_timestamp()`
already compiles to here, and it matches how the SQLite, MSSQL and
Oracle dialects handle `now()`.

Added `test_now_compiles_to_current_timestamp` in
`tests/unit/test_compiler.py`, which fails on `main` (`SELECT now() AS
ts`) and passes with this change. The package's unit tests pass locally,
and the noxfile's ruff 0.14.14 checks, flake8 and black report nothing
on the changed files.

- [x] Make sure to open an issue as a bug/issue before writing your code
(#15762)
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease

Co-authored-by: abhishekacharya200 <abhishekacharya200@users.noreply.github.com>

Author: abhishekacharya200

packages/sqlalchemy-bigquery/sqlalchemy_bigquery/base.py

@@ -588,6 +588,10 @@ def visit_not_regexp_match_op_binary(self, binary, operator, **kw):
     def visit_mod_binary(self, binary, operator, **kw):
         return f"MOD({self.process(binary.left, **kw)}, {self.process(binary.right, **kw)})"
 
+    def visit_now_func(self, fn, **kw):
+        # BigQuery has no NOW() function, use CURRENT_TIMESTAMP instead.
+        return "CURRENT_TIMESTAMP"
+
     def visit_try_cast(self, cast: elements.Cast, **kwargs: Any) -> str:
         """Compile a ``SAFE_CAST`` / ``try_cast`` expression for BigQuery."""
         return "SAFE_CAST(%s AS %s)" % (

packages/sqlalchemy-bigquery/tests/unit/test_compiler.py

@@ -491,6 +491,13 @@ def test_float_literal_pyformat_bindparam():
     assert "%(param_1:FLOAT64)s" in str(compiled)
 
 
+def test_now_compiles_to_current_timestamp(faux_conn):
+    # BigQuery has no NOW() function.
+    stmt = sqlalchemy.select(sqlalchemy.func.now().label("ts"))
+    found_sql = stmt.compile(faux_conn).string
+    assert found_sql == "SELECT CURRENT_TIMESTAMP AS `ts`"
+
+
 SAFE_CAST_TYPE_CASES = [
     pytest.param(sqlalchemy.Integer, "INT64", id="integer_class"),
     pytest.param(sqlalchemy.Integer(), "INT64", id="integer_instance"),