planner: KeyTooLong when inserting VECTOR with 3000 dimension

[breezewish]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE t(
    a VECTOR(3000),
    VECTOR INDEX ((VEC_COSINE_DISTANCE(a)))
);

INSERT INTO t VALUES ('....');

2. What did you expect to see? (Required)

Insert succeed.

3. What did you see instead (Required)

Error: KeyTooLong

4. What is your TiDB version? (Required)

v8.5

[breezewish]

Fixed by #58717