[PWGHF] treeCreatorOmegacSt: fix decayChannel shadowing and reset MC match variables

[chengtt0406]

Fix small bugs in processMc and fillTable

• processMc: reset member decayChannel, origin, and idxBhadMothers per MC particle; use decayChannel = -1 (no int) so the member is updated.
• fillTable: reset isMatched, origin, idxBhadMothers, indexRec, and indexRecCharmBaryon before MC matching for each candidate.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 0 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/o2 for 037fda5 at 2026-06-16 11:16:

## sw/BUILD/O2Physics-latest/log
c++: fatal error: Killed signal terminated program cc1plus
c++: fatal error: Killed signal terminated program cc1plus
ninja: build stopped: subcommand failed.

Full log here.

[vkucera]

Please, write a proper PR title and give details about the bug in the description.

[chengtt0406]

Please, write a proper PR title and give details about the bug in the description.

Hi, thanks for your comments. It is updated now.

[vkucera]

Please, write a proper PR title and give details about the bug in the description.

Hi, thanks for your comments. It is updated now.

Thanks but there is still no information about the location in the repo. Please see the contribution guidelines.

[chengtt0406]

Please, write a proper PR title and give details about the bug in the description.

Hi, thanks for your comments. It is updated now.

Thanks but there is still no information about the location in the repo. Please see the contribution guidelines.

Done :)