Keep subnormal plot ranges finite

[Kkkakania]

Summary

• clamp only overflowed ViewBox scale factors to the signed maximum finite float
• apply the same finite-scale guard when AxisItem maps extremely small ranges to tick coordinates
• cover positive and zero-crossing subnormal ranges on both horizontal and vertical axes

Root cause

For a nonzero range near 1e-310, dividing a normal pixel width or height by that range overflows to infinity. The resulting Qt transform contains inf/nan, and AxisItem generates non-finite tick endpoints. This can cause platform-dependent rendering failures or crashes.

A fixed epsilon would change the visible scale of legitimate small-valued data. This change instead preserves the requested range and clamps only an already-overflowed ratio, retaining its sign. Ordinary scales and zero-range handling are unchanged.

Tests

• focused ViewBox and AxisItem regression matrix: 8 passed
• QT_QPA_PLATFORM=offscreen python -m pytest tests -q: 1094 passed, 723 skipped, 8 xfailed
• repository pre-commit hooks for all four changed files: all applicable hooks passed; the removed upstream fix-encoding-pragma hook was skipped

Fixes #2370