Replace remaining `assert isinstance(...)` statements with proper `TypeError` exceptions across the codebase

[suhaan-24]

Following the pattern from #1033, I noticed there are still several assert isinstance(...) statements used for runtime type checking across the codebase. These have the same problem: they're silently disabled when Python runs with optimisation flags (python -O) and produce generic AssertionError instead of a descriptive TypeError.
Affected files and locations:

• frq_base.py — lines 275, 516
• cnv_frq.py — lines 93, 263, 507
• sample_metadata.py — lines 1167, 1280, 1329, 1431, 1695
• util.py — lines 314, 378, 649, 1474, 1475, 1495, 1496, 1515, 1516
  Each should be replaced with an explicit if not isinstance(...): raise TypeError(...) that includes a message about what type was expected vs. what was received.
  Happy to submit a PR for this!