refactor: reduce contour function complexity - functions exceed 50-line target

[krystophny]

Problem

Functions in fortplot_matplotlib.f90 exceed the 50-line target for maintainability:

• contour_filled() (lines 86-150): 65 lines
• add_contour_filled() (lines 441-505): 65 lines

Root Cause

The conditional parameter forwarding logic introduced in PR #402 creates extensive if/else chains to handle 16 possible parameter combinations safely.

Solution Options

1. Extract parameter forwarding logic to helper subroutine
2. Create parameter struct/type to reduce combinations
3. Use polymorphic interfaces to reduce branching

Impact

• Code maintainability affected by repetitive conditional chains
• Functions exceed QADS 50-line target (but within 100-line hard limit)
• Logic is correct but verbose

Priority

MINOR - Functions work correctly, refactoring improves maintainability only