__kitty_tab_set sent a luminance-picked black/white active_fg, so a
tinted tab's title looked different from a normal one. Send NONE
instead: tab_fg() then falls back to the shared active foreground (the
theme background) while active_bg keeps the per-host tint.
Covered by a new case in tests/test_tab_bar_colors.py matching what
__kitty_tab_set actually sends — host background, no fg override.
a68b138 made draw_tab() derive all four color roles from tab_colors(),
so it stopped reading the per-tab colors that `kitten @ set-tab-color`
sets. The ssh wrapper (__kitty_tab_set) still set them, but the renderer
discarded them and every remote tab drew in the #2e84e6 accent.
Resolve the drawn colors through DrawData.tab_fg/tab_bg on the already
role-substituted title_draw_data: those return the per-tab override when
one is set and the accent roles otherwise. Theme-provided tab colors
still cannot leak in, so the theme-independence fix is preserved.
Extend tests/test_tab_bar_colors.py with the override case; it fails
against the old renderer (got the accent, wanted the host color).
