feat #7713: implement ICARRV/CARRV as SymbolicRVs

[asifzubair]

Description

We will implement ICARRV and CARRV as SymbolicRVs by subclassing SymbolicMVNormalUsedInternally. This will eliminate the need for rng_fn.

Related Issue

• [x] Closes #7713
• [ ] Related to #

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
• [ ] 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):

---

📚 Documentation preview 📚: https://pymc--7879.org.readthedocs.build/en/7879/