fix(column): rebuild status column when sign column is invalidated (#22690)

* fix(column): rebuild status column when sign column is invalidated

Problem:    When implementing a custom sign column through
            `'statuscolumn'`, the status column is not properly rebuilt
            when the sign column width changes.
Solution:   Force a rebuild of the status column when the sign column
            width is invalidated.

* test(column): 'statuscolumn' has correct width when (un)placing signs

src/nvim/buffer.c

@@ -4109,6 +4109,7 @@ void buf_signcols_del_check(buf_T *buf, linenr_T line1, linenr_T line2)
 
   if (!buf->b_signcols.sentinel) {
     buf->b_signcols.valid = false;
+    invalidate_statuscol(NULL, buf);
     return;
   }
 
@@ -4117,6 +4118,7 @@ void buf_signcols_del_check(buf_T *buf, linenr_T line1, linenr_T line2)
   if (sent >= line1 && sent <= line2) {
     // Only invalidate when removing signs at the sentinel line.
     buf->b_signcols.valid = false;
+    invalidate_statuscol(NULL, buf);
   }
 }
 
@@ -4132,6 +4134,7 @@ void buf_signcols_add_check(buf_T *buf, sign_entry_T *added)
 
   if (!added || !buf->b_signcols.sentinel) {
     buf->b_signcols.valid = false;
+    invalidate_statuscol(NULL, buf);
     return;
   }