Result string is truncated to the fixed index length in LOOKUP clause

[Aiee]

Describe the bug (must be provided)

As title. This problem only exists in LOOKUP, and MATCH works well.
Detail: https://discuss.nebula-graph.com.cn/t/topic/3628/10

Your Environments (must be provided)

v2.5.0

How To Reproduce(must be provided)

Steps to reproduce the behavior:

CREATE TAG player(name string, age int);
CREATE TAG INDEX name ON player(name(20))
LOOKUP ON player YIELD player.name

[critical27]

Same with #2745, discuss over there, close for now.