[PWGEM] fix code-checker issues + add V0-photon track selection for photon pairs

[Steffimro]

• fixed the code-checker issues in the photonhbt.cxx task which popped up in the pull request ([PWGEM] Rewise phi-star separation cuts #17040)

• added a configurable for the photon pairs. This allows to select the leg track composition of the two
  V0 photons (counts of ITS-TPC, ITS-only, TPC-only, TRD, TOF legs per photon, class A or B). The selection is applied in same-event pairing and propagated to event mixing by storing the leg counts in the mixing pool. Disabled by default (cfgDoPhotonClassPairCut = false).

[github-actions]

O2 linter results: ❌ 185 errors, ⚠️ 332 warnings, 🔕 3 disabled

[alibuild]

Error while checking build/O2Physics/code-check for d8479d0 at 2026-07-20 15:10:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 5 errors and 0 warnings.'
Found 5 errors and 0 warnings.
++ [[ 5 -gt 0 ]]
++ cat /sw/BUILD/5c26216a310a34b6fdb09257f3d4e711c3f39e13/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/Core/Pi0EtaToGammaGammaMC.h:268:25: error: no header providing "std::array" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Core/Pi0EtaToGammaGamma.h:768:18: error: no header providing "std::array" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Tasks/photonhbt.cxx:117:8: error: constructor does not initialize these fields: ccdb [cppcoreguidelines-pro-type-member-init,-warnings-as-errors]
PWGEM/PhotonMeson/Tasks/photonhbt.cxx:205:3: error: no header providing "o2::framework::Service" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/PairUtilities.h:38:1: error: initial values in enum 'o2::aod::pwgem::photonmeson::photonpair::PairType' are not consistent, consider explicit initialization of all, none or only the first enumerator [readability-enum-initial-value,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 5 -gt 0 ]]
++ exit 1
--

Full log here.

[alibuild]

Error while checking build/O2Physics/code-check for 560fae0 at 2026-07-20 17:23:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 2 errors and 0 warnings.'
Found 2 errors and 0 warnings.
++ [[ 2 -gt 0 ]]
++ cat /sw/BUILD/6c1d2e709410d7d507c57bd7fd18b18ef2a1da1a/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/Tasks/photonhbt.cxx:205:3: error: no header providing "o2::framework::Service" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/PairUtilities.h:22:1: error: included header AnalysisHelpers.h is not used directly [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 2 -gt 0 ]]
++ exit 1
--

Full log here.