Implemented icdf for Categorical distribution and add shape/broadcasting tests.

[iTzZe4lous]

Description

Changes

• Added icdf implementation to pymc/distributions/discrete.py.
• Added test_categorical_icdf, test_categorical_icdf_batch_shapes, and test_categorical_icdf_upper_edge.

Testing

• All Categorical tests pass locally (pytest -q tests/distributions -k categorical).
• Pre-commit checks pass.

Related Issue

• [ ] Closes #
• [x] Related to # 6612

Checklist

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

Type of change

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

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📚 Documentation preview 📚: https://pymc--7920.org.readthedocs.build/en/7920/