sql/sem/tree: fix width handling for BIT column default values

Previously, when a column was being added or dropped and had NOT NULL
with no default value, the parseComputedExpr logic would generate a
placeholder value to avoid NULL violations (see #46285 for historical
context). For columns of type BIT(n), this fallback value was
incorrectly set as BIT(0), which has a width mismatch and caused errors
during row insertions or deletions.

This change ensures that the fallback value matches the column's
specified BIT(n) width, allowing operations to proceed without errors
during schema changes involving such columns.

Fixes #152326

Release note (bug fix): Fixed a bug where INSERTs could fail with a type
checking error while adding a BIT(n) column.

Epic: None

Author: spilchen

pkg/ccl/schemachangerccl/backup_base_generated_test.go

@@ -0,0 +1,31 @@
+setup
+CREATE TABLE bittab (c1 INT);
+----
+
+# Insert is blocked prior to setting the NULL attribute on the column due to a
+# temporary constraint.
+stage-exec phase=PostCommitPhase stage=4
+INSERT INTO bittab(c1) VALUES (1);
+----
+pq: failed to satisfy CHECK constraint .*
+
+# At this stage, the NULL attribute is now added to the column and temp
+# constraint is dropped. We allow the INSERT. A suitable value for the new
+# column is selected in tree.NewDefaultDatum.
+stage-exec phase=PostCommitPhase stage=5
+INSERT INTO bittab(c1) VALUES (1);
+----
+
+stage-query phase=PostCommitPhase stage=6
+SELECT count(*) FROM bittab
+----
+1
+
+# Confirm that we can delete the new column value.
+stage-exec phase=PostCommitPhase stage=7
+DELETE FROM bittab
+----
+
+test
+ALTER TABLE bittab ADD COLUMN vb VARBIT(25) NOT NULL UNIQUE, ADD COLUMN fb BIT(25) NOT NULL
+----