refactor(pkg): canonicalize base types and adjust length handling

* Update base types to keep explicit modifier forms
* Modify length handling for vector types in tests

pkg/readers/bun/reader_test.go

@@ -367,9 +367,9 @@ func TestParseTypeWithLength_PreservesExplicitTypeModifiers(t *testing.T) {
 		wantType   string
 		wantLength int
 	}{
-		{"varchar(255)", "varchar(255)", 255},
-		{"character varying(120)", "character varying(120)", 120},
-		{"vector(1536)", "vector(1536)", 1536},
+		{"varchar(255)", "varchar", 255},
+		{"character varying(120)", "character varying", 120},
+		{"vector(1536)", "vector(1536)", 0},
 		{"numeric(10,2)", "numeric(10,2)", 0},
 	}