feat(extmarks): add sign name to extmark "details" array

Problem:  Unable to identify legacy signs when fetching extmarks with
          `nvim_buf_get_extmarks()`.
Solution: Add "sign_name" to the extmark detail array.

Add some misc. changes as follow-up to #25724

runtime/doc/builtin.txt

@@ -6998,11 +6998,11 @@ sign_place({id}, {group}, {name}, {buf} [, {dict}])               *sign_place()*
 		similar to the |:sign-place| command.
 
 		If the sign identifier {id} is zero, then a new identifier is
-		allocated.  Otherwise the specified number is used. {group} is
+		allocated (first available id in a buffer).  Otherwise the
-		the sign group name. To use the global sign group, use an
+		specified number is used. {group} is the sign group name. To use
-		empty string.  {group} functions as a namespace for {id}, thus
+		the global sign group, use an empty string.  {group} functions
-		two groups can use the same IDs. Refer to |sign-identifier|
+		as a namespace for {id}, thus two groups can use the same IDs.
-		and |sign-group| for more information.
+		Refer to |sign-identifier| and |sign-group| for more information.
 
 		{name} refers to a defined sign.
 		{buf} refers to a buffer name or number. For the accepted

runtime/doc/sign.txt

@@ -53,9 +53,9 @@ If 'cursorline' is enabled, then the CursorLineSign highlight group is used
 Each placed sign is identified by a number called the sign identifier. This
 identifier is used to jump to the sign or to remove the sign. The identifier
 is assigned when placing the sign using the |:sign-place| command or the
-|sign_place()| function. Each sign identifier should be a unique number.
+|sign_place()| function. Each sign identifier should be a unique number (per
-Placing the same identifier twice will move the previously placed sign. The
+buffer). Placing the same identifier twice will move the previously placed sign.
-|sign_place()| function can be called with a zero sign identifier to allocate
+The |sign_place()| function can be called with a zero sign identifier to allocate
 the next available identifier.
 
 							*sign-group*

src/nvim/decoration.c

@@ -1022,6 +1022,10 @@ void decor_to_dict_legacy(Dictionary *dict, DecorInline decor, bool hl_name)
       PUT(*dict, "sign_text", CSTR_TO_OBJ(sh_sign.text.ptr));
     }
 
+    if (sh_sign.sign_name) {
+      PUT(*dict, "sign_name", CSTR_TO_OBJ(sh_sign.sign_name));
+    }
+
     // uncrustify:off
 
     struct { char *name; const int val; } hls[] = {

src/nvim/sign.c

@@ -171,12 +171,12 @@ int sign_cmp(const void *p1, const void *p2)
          ? n : (sh2->sign_add_id - sh1->sign_add_id);
 }
 
-/// Delete the specified signs
+/// Delete the specified sign(s)
 ///
 /// @param buf  buffer sign is stored in or NULL for all buffers
 /// @param group  sign group
 /// @param id  sign id
-/// @param atlnum  sign at this line, -1 at any line
+/// @param atlnum  single sign at this line, specified signs at any line when -1
 static int buf_delete_signs(buf_T *buf, char *group, int id, linenr_T atlnum)
 {
   int64_t ns = group_get_ns(group);
@@ -187,7 +187,7 @@ static int buf_delete_signs(buf_T *buf, char *group, int id, linenr_T atlnum)
   MarkTreeIter itr[1];
   int row = atlnum > 0 ? atlnum - 1 : 0;
   kvec_t(MTKey) signs = KV_INITIAL_VALUE;
-  // Store and sort when removing a single sign at a specific line number.
+  // Store signs at a specific line number to remove one later.
   if (atlnum > 0) {
     if (!marktree_itr_get_overlap(buf->b_marktree, row, 0, itr)) {
       return FAIL;
@@ -222,6 +222,7 @@ static int buf_delete_signs(buf_T *buf, char *group, int id, linenr_T atlnum)
     }
   }
 
+  // Sort to remove the highest priority sign at a specific line number.
   if (kv_size(signs)) {
     qsort((void *)&kv_A(signs, 0), kv_size(signs), sizeof(MTKey), sign_cmp);
     extmark_del_id(buf, kv_A(signs, 0).ns, kv_A(signs, 0).id);
@@ -1198,11 +1199,7 @@ static int sign_define_from_dict(char *name, dict_T *dict)
     numhl = tv_dict_get_string(dict, "numhl", false);
   }
 
-  if (sign_define_by_name(name, icon, text, linehl, texthl, culhl, numhl) == OK) {
+  return sign_define_by_name(name, icon, text, linehl, texthl, culhl, numhl) - 1;
-    return 0;
-  }
-
-  return -1;
 }
 
 /// Define multiple signs using attributes from list 'l' and store the return
@@ -1563,7 +1560,7 @@ static int sign_unplace_from_dict(typval_T *group_tv, dict_T *dict)
     }
   }
 
-  return sign_unplace(buf, id, group, 0) ? 0 : -1;
+  return sign_unplace(buf, id, group, 0) - 1;
 }
 
 /// "sign_unplace()" function

test/functional/api/extmark_spec.lua

@@ -1582,6 +1582,23 @@ describe('API/extmarks', function()
       priority = 4096,
       right_gravity = true,
     } }, get_extmark_by_id(ns, marks[3], { details = true }))
+    curbufmeths.clear_namespace(ns, 0, -1)
+    -- legacy sign mark includes sign name
+    command('sign define sign1 text=s1 texthl=Title linehl=LineNR numhl=Normal culhl=CursorLine')
+    command('sign place 1 name=sign1 line=1')
+    eq({ {1, 0, 0, {
+      cursorline_hl_group = 'CursorLine',
+      invalidate = true,
+      line_hl_group = 'LineNr',
+      ns_id = 0,
+      number_hl_group = 'Normal',
+      priority = 10,
+      right_gravity = true,
+      sign_hl_group = 'Title',
+      sign_name = 'sign1',
+      sign_text = 's1',
+      undo_restore = false
+    } } }, get_extmarks(-1, 0, -1, { details = true }))
   end)
 
   it('can get marks from anonymous namespaces', function()