Fix traceback colors shared between logging handlers

[nk-batman21]

Fixes #143.

On Python 3.13+, the colored traceback is cached on the shared LogRecord. A later plain formatter reuses its ANSI escapes; running the plain formatter first prevents the colored formatter from coloring the traceback.

Keep plain exception text on the original record for record-forwarding callers, and render each formatter's exception colors on a local copy with a fresh cache. Records that arrive with cached exception text and no exc_info retain that text.

Tests cover both handler orders with standard, no_color and non-TTY formatters, plus pickled records with exception information removed. Seven new regressions fail before the fix; the two cached-text-only controls pass before and after.

Validation:

• Full pytest suite on Python 3.8 through 3.14: 33 tests on each older version, 44 on Python 3.13 and 3.14.
• Tox black, flake8, mypy and py environments pass.
• Whole-project Black, Flake8 and mypy checks on Python 3.14, and git diff --check, pass.