MDEV-30354 Fix JSON escaping in optimizer trace

[y2kwak]

Description

Currently, when using optimizer trace, the expanded_query field can produce invalid JSON when queries contain special characters like quotes or backslashes. This makes the JSON output invalid and difficult to parse programmatically.

This PR modifies the expanded_query output to properly escape special characters using json_escape(), ensuring valid JSON output while maintaining readability.

It also includes MTR tests to verify the fix.

How can this PR be tested?

Enable optimizer trace and run queries with special characters:

SET optimizer_trace='enabled=on';
SELECT LENGTH("a");
SELECT json_valid(trace), trace FROM information_schema.optimizer_trace;

Before change:

{
            "expanded_query": "select octet_length('a') AS `LENGTH("a")`"
}

After change:

{
            "expanded_query": "select octet_length('a') AS `LENGTH(\"a\")`"
}

Additional test cases:

• Nested quotes: SELECT LENGTH("a\"b");

{
    "expanded_query": "select octet_length('a\"b') AS `LENGTH(\"a\\\"b\")`"
}

• Special characters with quotes: SELECT LENGTH("a\n\"b\t\"c");

{
    "expanded_query": "select octet_length('a\\n\"b\\t\"c') AS `LENGTH(\"a\\n\\\"b\\t\\\"c\")`"
}

• New line:

SELECT COLUMN_JSON(COLUMN_CREATE("foo", "New
line" AS CHAR));

{
    "expanded_query": "select column_json(column_create('foo','New\\nline' AS char charset latin1 collate latin1_swedish_ci )) AS `COLUMN_JSON(COLUMN_CREATE(\"foo\", \"New\nline\" AS CHAR))`"}

Basing the PR against the correct MariaDB version

• This is a new feature and the PR is based against the latest MariaDB development branch.

Copyright

All new code of the whole pull request, including one or several files that are either new files or modified ones, are contributed under the BSD-new license. I am contributing on behalf of my employer Amazon Web Services, Inc.

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[svoj]

@y2kwak thanks for your contribution! A few minor issues outlined.