MDEV-40486 Length check for vector fields in CREATE TABLE ... SELECT

[mariadb-YuchenPei]

The changes of MDEV-39558 2b65294 added length check assertion in Field_varstring constructors, and length check in type inference for SELECT set operations, to emit errors before reaching the assertions.

That change caused an error to turn into an assertion failure in a separate path, when the length limit violation is not detected before tripping the assertion. So in this patch we fix it by adding an earlier length check in that path.

The reason that we place this check inside
Item_func_vec_fromtext::fix_length_and_dec rather than say `create_field_for_create_select is for consistency:

If

create table t1 as select
vec_fromtext(concat('[',group_concat(1),']')) as c1 from seq_1_to_64;

fails due to length limit violation, then so should

create table t1 (v vector(64) not null);
insert into t1 select vec_fromtext(concat('[',group_concat(1),']')) from seq_1_to_64;

Also use max_char_length() instead of max_length. This is a more accurate length of characters. And add handling of empty string edge case. Added testcases accordingly.

The change that uses max_char_length() causes side effects where creating a table using a VEC_FROMTEXT(CHAR(1)) would result in a 0-dimensional vector field. This is accurate but 0-dim vector table fields should not be allowed. So we make cases like this result in a one-dimensional field.

Also fixed the underflow in (args[0]->max_length - 1) * 2 when the arg's max length is 0. Previously this underflow would cause

create table t1 select vec_fromtext(NULL)

to fail with ER_TOO_BIG_FIELDLENGTH. Now it will be a VECTOR(1) field

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[mariadb-YuchenPei]

This is an alternative patch to #5443