Fix LF_rapidity generator by including generator_pythia8_longlived.C

[sawankumawat]

• Include generator_pythia8_longlived.C to access getMass() function
• Replace custom getMassFromPDG helper with GeneratorPythia8LongLivedGun::getMass()
• Aligns with working generator_pythia8_LF.C structure
• Created a gun for exotic particles

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
To request your PR to be included in production software, please add the corresponding labels called "async-" to your PR. Add the labels directly (if you have the permissions) or add a comment of the form (note that labels are separated by a ",")

+async-label <label1>, <label2>, !<label3> ...

This will add <label1> and <label2> and removes <label3>.

The following labels are available
async-2023-pbpb-apass4
async-2023-pp-apass4
async-2024-pp-apass1
async-2022-pp-apass7
async-2024-pp-cpass0
async-2024-PbPb-apass1
async-2024-ppRef-apass1
async-2024-PbPb-apass2
async-2023-PbPb-apass5

[njacazio]

Ciao @sawankumawat I see some formatting changes, maybe add those in a separate PR

[sawankumawat]

Ciao @sawankumawat I see some formatting changes, maybe add those in a separate PR

Thanks for reviewing the request. I will make another commit without the formatting changes.

[sawankumawat]

Ciao @sawankumawat I see some formatting changes, maybe add those in a separate PR

Thanks for reviewing the request. I will make another commit without the formatting changes.

Removed the formatting changes.