refactor: replace cast() with isinstance assertions in reports

[ovodela]

Summary

• Replaces all 4 cast() calls in dataprofiler/reports/graphs.py with assert isinstance() or assert is not None checks
• Provides both static type safety (mypy narrowing) and runtime validation instead of unchecked casts
• Removes unused Union and cast imports from typing
• The cast(F, new_f) in utils.py is intentionally retained -- it handles a fundamental typing limitation with decorator return types that cannot be expressed with isinstance

Changes

Location	Before	After
Line 77	cast(list[Union[str, int]], column_names)	assert column_names is not None
Line 100	cast(ColumnPrimitiveTypeProfileCompiler, ...)	assert isinstance(data_compiler, ...)
Line 130	Same cast	Same assert
Line 269	cast(Figure, ax.figure)	assert ax.figure is not None

Test plan

• All existing test_graphs.py tests pass (16 tests)
• mypy reports no new type errors
• No behavioral changes -- assertions are satisfied on all existing code paths

Addresses #720

[CLAassistant]

All committers have signed the CLA.

[ovodela]

Closing this PR — re-submitting through the Capital One Spork contribution pipeline (via capitalone-contributions fork) for proper compliance. Same changes will be in the new PR.