planner: fix a bug that point get plan returns wrong column name

[xiongjiwei]

What problem does this PR solve?

Issue Number: close #23291

as #23291 (comment) says, the results of TiDB are problematic
also, we explicitly select the column name and expect to get the result with the same column name.

Check List

Tests

• Manual test (add detailed scripts or steps below)

create table t1(`COL1` varchar(255), PRIMARY KEY (`COL1`));
insert into t1 values("a");
SELECT col1 FROM t1 where col1="a";
+------+
| col1 |    -- it should be lower case
+------+
| a    |
+------+
1 row in set

Release note

• fix a bug that point get plan returns wrong column name

[xiongjiwei]

/run-all-tests

[xiongjiwei]

/cc @winoros

[djshow832]

Please add test cases.

[xiongjiwei]

Please add test cases.

it is hard to use our test framework to test the result column name, I have tried to enhance it, but it will export a lot of variables and changes a lot. So I add a Manual test

[djshow832]

/lgtm

[eurekaka]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• djshow832
• eurekaka

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[xiongjiwei]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ee42cd4

[xiongjiwei]

/run-unit-test

[lysu]

@xiongjiwei maybe need cherry-pick to 4.0-5.0 some user need it in oncall

[ti-srebot]

cherry pick to release-5.0 in PR #23969

[ti-srebot]

cherry pick to release-4.0 in PR #23970