[ruby/error_highlight] Return nil instead of raising NotImplementedError for def/lambda/block args

ErrorHighlight.spot defaults point_type to :args for ArgumentError. When an
ArgumentError originates from argument binding (e.g. a missing required
keyword), its first backtrace location maps to a def/lambda/block node. The
spotter had no implementation for the :args point type on those nodes and did
`raise NotImplementedError`.

This is a regression from 0.7.0 (and in Ruby 4.0 compared to Ruby 3.4).

ErrorHighlight.spot only rescues SyntaxError/SystemCallError/ArgumentError, and
NotImplementedError is not a StandardError, so it escaped through
CoreExt#generate_snippet -> Exception#detailed_message / #full_message and
replaced the original exception. This is normally hidden because the VM's
"missing keyword" message matches the keyword regex in generate_snippet and
takes the safe :name branch, but it surfaces as soon as the ArgumentError is
re-raised/wrapped with a custom message that falls into the :args else branch.

Return nil (the documented "no spot" result) for these cases instead, matching
the graceful-degradation behavior of the surrounding feature. The fix is
applied consistently across both compilers:
  - prism:     def_node / lambda_node / block_node
  - parse.y:   DEFN / DEFS (raised NotImplementedError) and LAMBDA / ITER
               (silently returned a spot for :args, inconsistent with prism)

All of these branches were introduced together by the experimental "wrong
number of arguments" snippet feature and are only intended for point_type
:name, so returning nil for any other point_type keeps the two parsers in sync.

ruby/error_highlight@ca126db2db

[Bug #22129] (Backport for 4.0)
(cherry picked from 12ef8a6)

Author: rwstauner

lib/error_highlight/base.rb

@@ -235,17 +235,21 @@ def spot
         spot_op_cdecl
 
       when :DEFN
-        raise NotImplementedError if @point_type != :name
+        # There is nothing to highlight for the arguments of a method
+        # definition, so just return nil instead of raising.
+        return nil if @point_type != :name
         spot_defn
 
       when :DEFS
-        raise NotImplementedError if @point_type != :name
+        return nil if @point_type != :name
         spot_defs
 
       when :LAMBDA
+        return nil if @point_type != :name
         spot_lambda
 
       when :ITER
+        return nil if @point_type != :name
         spot_iter
 
       when :call_node
@@ -294,23 +298,30 @@ def spot
         when :name
           prism_spot_def_for_name
         when :args
-          raise NotImplementedError
+          # There is nothing to highlight for the arguments of a method
+          # definition (e.g. an ArgumentError for a missing required keyword
+          # is spotted with point_type: :args). Return nil instead of raising
+          # NotImplementedError, which would otherwise escape
+          # Exception#detailed_message / #full_message.
+          return nil
         end
 
       when :lambda_node
         case @point_type
         when :name
           prism_spot_lambda_for_name
         when :args
-          raise NotImplementedError
+          # See the comment for :def_node above.
+          return nil
         end
 
       when :block_node
         case @point_type
         when :name
           prism_spot_block_for_name
         when :args
-          raise NotImplementedError
+          # See the comment for :def_node above.
+          return nil
         end
 
       end

test/error_highlight/test_error_highlight.rb

@@ -1751,6 +1751,69 @@ def test_singleton_method_multiple_missing_keywords
     end
   end
 
+  def def_with_required_keyword(x:)
+    x
+  end
+
+  def test_spot_with_args_point_type_on_def_node_returns_nil
+    # A method with a required keyword argument called without it raises an
+    # ArgumentError whose first backtrace location maps to the `def` node.
+    # ErrorHighlight.spot defaults point_type to :args for ArgumentError, and
+    # there is no way to highlight "the arguments" of a definition, so spot
+    # must return nil instead of raising NotImplementedError.
+    begin
+      def_with_required_keyword
+    rescue ArgumentError => exc
+    end
+
+    assert_nil(ErrorHighlight.spot(exc))
+  end
+
+  def test_spot_with_args_point_type_on_lambda_node_returns_nil
+    l = lambda { |x:| x }
+    begin
+      l.call
+    rescue ArgumentError => exc
+    end
+
+    assert_nil(ErrorHighlight.spot(exc))
+  end
+
+  define_method(:block_with_required_keyword) { |x:| x }
+
+  def test_spot_with_args_point_type_on_block_node_returns_nil
+    begin
+      block_with_required_keyword
+    rescue ArgumentError => exc
+    end
+
+    assert_nil(ErrorHighlight.spot(exc))
+  end
+
+  def test_detailed_message_does_not_raise_when_argument_error_is_rewrapped
+    # This reproduces a real-world crash: an ArgumentError originating from a
+    # missing required keyword (which maps to the `def` node) is re-raised with
+    # a custom message. The custom message no longer matches the keyword regex
+    # in CoreExt#generate_snippet, so the :args branch is taken. The spotter
+    # must not raise NotImplementedError (which is not a StandardError and would
+    # escape detailed_message / full_message).
+    begin
+      def_with_required_keyword
+    rescue ArgumentError => original
+      exc = original.exception("a custom message that is not a keyword error")
+    end
+
+    msg = nil
+    assert_nothing_raised do
+      msg = exc.detailed_message(highlight: false)
+    end
+    assert_match("a custom message that is not a keyword error", msg)
+
+    assert_nothing_raised do
+      exc.full_message(highlight: false)
+    end
+  end
+
   private
 
   def find_node_by_id(node, node_id)