ansi-color: $SYSTEMD_COLORS=true should mean generic "auto", not 24bit

Follow-up for af718e0535

The commit changed the documentation, but the actual impl
was apparently not updated. And the documented behavior
feels a bit off. I think generally "auto"/true should
override $NO_COLOR.

Plus, the test for auto-24bit is at odds with the logic
we merged. I guess it was overlooked after applying
https://github.com/systemd/systemd/pull/40303#discussion_r2720450393

src/basic/ansi-color.c

@@ -59,7 +59,7 @@ static ColorMode get_color_mode_impl(void) {
                 return m;
 
         /* Next, check for the presence of $NO_COLOR; value is ignored. */
-        if (getenv("NO_COLOR"))
+        if (m != COLOR_TRUE && getenv("NO_COLOR"))
                 return COLOR_OFF;
 
         /* If the above didn't work, we turn colors off unless we are on a TTY. And if we are on a TTY we
@@ -108,9 +108,10 @@ static const char* const color_mode_table[_COLOR_MODE_MAX] = {
         [COLOR_AUTO_16]    = "auto-16",
         [COLOR_AUTO_256]   = "auto-256",
         [COLOR_AUTO_24BIT] = "auto-24bit",
+        [COLOR_TRUE]       = "true",
 };
 
-DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(color_mode, ColorMode, COLOR_24BIT);
+DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(color_mode, ColorMode, COLOR_TRUE);
 
 /*
  * Check that the string is formatted like an ANSI color code, i.e. that it consists of one or more

src/basic/ansi-color.h

@@ -17,6 +17,9 @@ typedef enum ColorMode {
         COLOR_AUTO_256,
         COLOR_AUTO_24BIT,
 
+        /* Same as default (unset), except that $NO_COLOR is ignored/overridden */
+        COLOR_TRUE,
+
         _COLOR_MODE_MAX,
         _COLOR_MODE_INVALID = -EINVAL,
 } ColorMode;