fix: replace assert isinstance(...) statements with proper TypeError exceptions across codebase

[suhaan-24]

Summary

This PR replaces 13 assert isinstance(...) statements across 4 files with explicit if not isinstance(...): raise TypeError(...) blocks, following the same pattern established in #1033.

Why this matters

Python's assert statements are silently removed when running with optimisation flags (python -O), making them unreliable for runtime type checking. The new TypeError exceptions include descriptive messages showing the expected vs actual type, making debugging easier for users.

Changes

• frq_base.py — replaced 2 type assertions in plotting functions
• cnv_frq.py — replaced 3 type assertions in gene CNV frequency methods
• sample_metadata.py — replaced 5 type assertions in sample lookup and cohort setup methods
• util.py — replaced 3 type assertions in internal helpers

Verification

All 150 relevant tests pass, and there are no changes to behaviour for valid inputs.
Closes #1041

[suhaan-24]

Hey @jonbrenas all checks are passed please have a look at this