MDEV-8235: Expose check_slave_start_position as SQL function GTID_CHECK_POS()

[ayush-jha123]

I’ve been diving deep into the replication source code for the last two weeks,
investigating MDEV-8235. I noticed this task has been open for a while and realized
how useful it would be for DBAs to have a simple way to check GTID availability
directly from the SQL layer.

After studying how check_slave_start_position() works internally, I’ve implemented
a user-facing version called GTID_CHECK_POS().

What this PR does:

• Extracted the internal GTID binlog-lookup logic into a shared helper function
  in sql_repl.cc to avoid code duplication.
• Created the Item_func_gtid_check_pos class to wrap the logic for the SQL parser.
• Added input validation: it returns 1 (exists), 0 (purged), or NULL (invalid input)
  which makes it robust for automation scripts.
• Included an MTR test main.gtid_check_pos covering standard and edge cases.

I’m really excited to contribute this to MariaDB and would appreciate feedback
on the implementation.

[gkodinov]

Thank you for your contribution! This is a preliminary review.

First, the formalities:

• please keep a single commit, with a commit message in it.
• please make sure all of the buildbot hosts compile and run successfully

Below are some of my thoughts on the functionality and its implementation and test coverage. You can ignore all of the below and take it with the final reviewer. Or address it and have a faster final review with these fixed.

[gkodinov]

It's looking much better. Please make sure to address the comments below. This is my last batch of preliminary review comments.
Thanks for your efforts.

[gkodinov]

no space only changes please.

[gkodinov]

revert these please

[gkodinov]

revert this please.

[gkodinov]

This fails to compile on buildbot (I'm assuming for the unit tests) as follows:

/home/buildbot/aarch64-debian-11/build/sql/item_func.cc: In member function ‘virtual bool Item_func_gtid_check_pos::val_bool()’:
/home/buildbot/aarch64-debian-11/build/sql/item_func.cc:841:7: error: ‘rpl_gtid_pos_check_reachable’ was not declared in this scope
  841 |   if (rpl_gtid_pos_check_reachable(gtid_str, &is_reachable))
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Please have a look.

[ayush-jha123]

Hi @gkodinov,

I've just pushed the updated commit addressing your latest feedback:

Compilaton: Added #include "sql_repl.h" to sql/item_func.cc to fix the scope error you observed on Buildbot.
Submodules: Reverted all changes to the libmariadb and storage/columnstore/columnstore submodules.
Whitespace: I've ensured that no space-only changes are included in the diff; the comment lines and empty lines now match the original source exactly.

Thanks for the review, let me know if there's anything else!