Added icdf for HalfStudentT; tests

[dalai444]

Description

Wrote ICDF for HalfStudentT as well as relevant tests.

Related Issue

• Closes #
• [ x] Related to Add icdf functions for distributions #6612

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7919.org.readthedocs.build/en/7919/

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