fix: render assistant replies with disabled colors - #5893

Fixes #5873.

An invalid assistant_output_color is reset to None by color validation. Both assistant reply renderers then pass it to Rich Markdown, which raises AttributeError before the model's edits can be applied. Use Rich's default "none" style for disabled colors in the streaming and regular renderers, while preserving configured valid colors.

Validation:

• Added real Rich-rendering regressions for invalid, None, empty, and valid colors in both renderers. The original implementation fails four subtests; all eight pass with the fix.
• tests/basic/test_io.py and tests/basic/test_coder.py: 66 passed and 37 subtests passed. The existing test_color_initialization fails in this non-interactive Windows environment because PromptSession requires a Windows console; the same test also fails on the unchanged base checkout.
• Six offline Coder.run scenarios use LiteLLM mock responses with streaming enabled/disabled and invalid/disabled/valid colors. Every scenario renders the reply and applies its edit to a real local file.
• All applicable repository pre-commit hooks passed: isort, Black, flake8, and codespell.