fix(discussion/shared): error on out-of-range discussion number in URL

The discussion URL pattern captures one or more digits, which can exceed
the int32 range. The parse error from strconv.ParseInt was previously
discarded, silently clamping the number. Return an explicit error
instead, and cover it with tests in both URL parsing functions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: babakks

pkg/cmd/discussion/shared/lookup.go

@@ -38,7 +38,11 @@ func ParseDiscussionArg(arg string) (int32, ghrepo.Interface, error) {
 		return 0, nil, fmt.Errorf("invalid discussion URL: %q", arg)
 	}
 
-	num, _ := strconv.ParseInt(m[3], 10, 32)
+	num, err := strconv.ParseInt(m[3], 10, 32)
+	if err != nil {
+		return 0, nil, fmt.Errorf("invalid discussion number in URL: %q", m[3])
+	}
+
 	repo := ghrepo.NewWithHost(m[1], m[2], u.Hostname())
 	return int32(num), repo, nil
 }
@@ -84,7 +88,10 @@ func ParseDiscussionOrCommentArg(arg string) (*ParsedDiscussionOrCommentArg, err
 		return nil, fmt.Errorf("invalid discussion URL: %q", arg)
 	}
 
-	num, _ := strconv.ParseInt(m[3], 10, 32)
+	num, err := strconv.ParseInt(m[3], 10, 32)
+	if err != nil {
+		return nil, fmt.Errorf("invalid discussion number in URL: %q", m[3])
+	}
 	repo := ghrepo.NewWithHost(m[1], m[2], u.Hostname())
 
 	if fragment := u.Fragment; strings.HasPrefix(fragment, "discussioncomment-") {

pkg/cmd/discussion/shared/lookup_test.go

@@ -52,6 +52,11 @@ func TestParseDiscussionArg(t *testing.T) {
 			arg:     "https://github.com/owner/repo/discussions/",
 			wantErr: `invalid discussion URL: "https://github.com/owner/repo/discussions/"`,
 		},
+		{
+			name:    "URL with overflowing number",
+			arg:     "https://github.com/owner/repo/discussions/99999999999999999999",
+			wantErr: `invalid discussion number in URL: "99999999999999999999"`,
+		},
 		{
 			name:    "zero",
 			arg:     "0",
@@ -166,6 +171,11 @@ func TestParseDiscussionOrCommentArg(t *testing.T) {
 			arg:     "https://github.com/owner/repo/discussions/",
 			wantErr: `invalid discussion URL: "https://github.com/owner/repo/discussions/"`,
 		},
+		{
+			name:    "URL with overflowing number",
+			arg:     "https://github.com/owner/repo/discussions/99999999999999999999",
+			wantErr: `invalid discussion number in URL: "99999999999999999999"`,
+		},
 		{
 			name:    "comment URL with invalid fragment",
 			arg:     "https://github.com/owner/repo/discussions/5#discussioncomment-abc",