fix: case-insensitive Link rel="alternate" per RFC 8288

MANIFEST

@@ -66,6 +66,7 @@ t/Net-ACME2-get_orders.t
 t/Net-ACME2-HTTP.t
 t/Net-ACME2-JWTMaker.t
 t/Net-ACME2-key-change.t
+t/Net-ACME2-link-alternates.t
 t/Net-ACME2-order-lifecycle.t
 t/Net-ACME2-PromiseUtil.t
 t/Net-ACME2-RetryAfter.t

lib/Net/ACME2.pm

@@ -923,7 +923,7 @@ sub _parse_link_alternates {
 
     my @alt_urls;
     for my $link (@links) {
-        if ($link =~ m{<([^>]+)>\s*;\s*rel="alternate"}) {
+        if ($link =~ m{<([^>]+)>\s*;\s*rel="alternate"}i) {
             push @alt_urls, $1;
         }
     }

t/Net-ACME2-link-alternates.t

@@ -0,0 +1,128 @@
+#!/usr/bin/env perl
+
+use strict;
+use warnings;
+
+use Test::More;
+use Test::FailWarnings;
+
+# _parse_link_alternates is a private function in Net::ACME2.
+# We test it directly to verify RFC 8288 compliance (case-insensitive
+# relation types).
+
+use Net::ACME2 ();
+
+{
+    package MockResponse;
+
+    sub new {
+        my ($class, %opts) = @_;
+        return bless \%opts, $class;
+    }
+
+    sub header {
+        my ($self, $name) = @_;
+        return $self->{ lc $name };
+    }
+}
+
+# --- No Link header ---
+{
+    my $resp = MockResponse->new();
+    my @urls = Net::ACME2::_parse_link_alternates($resp);
+    is( scalar @urls, 0, 'no Link header returns empty list' );
+}
+
+# --- Single alternate link (lowercase) ---
+{
+    my $resp = MockResponse->new(
+        link => '<https://ca.example/cert/alt1>;rel="alternate"',
+    );
+    my @urls = Net::ACME2::_parse_link_alternates($resp);
+    is_deeply(
+        \@urls,
+        ['https://ca.example/cert/alt1'],
+        'single lowercase alternate link',
+    );
+}
+
+# --- Multiple alternate links (arrayref) ---
+{
+    my $resp = MockResponse->new(
+        link => [
+            '<https://ca.example/cert/alt1>;rel="alternate"',
+            '<https://ca.example/cert/alt2>;rel="alternate"',
+        ],
+    );
+    my @urls = Net::ACME2::_parse_link_alternates($resp);
+    is_deeply(
+        \@urls,
+        ['https://ca.example/cert/alt1', 'https://ca.example/cert/alt2'],
+        'multiple alternate links as arrayref',
+    );
+}
+
+# --- Non-alternate link is ignored ---
+{
+    my $resp = MockResponse->new(
+        link => '<https://ca.example/directory>;rel="index"',
+    );
+    my @urls = Net::ACME2::_parse_link_alternates($resp);
+    is( scalar @urls, 0, 'non-alternate link ignored' );
+}
+
+# --- Mixed alternate and non-alternate ---
+{
+    my $resp = MockResponse->new(
+        link => [
+            '<https://ca.example/directory>;rel="index"',
+            '<https://ca.example/cert/alt1>;rel="alternate"',
+        ],
+    );
+    my @urls = Net::ACME2::_parse_link_alternates($resp);
+    is_deeply(
+        \@urls,
+        ['https://ca.example/cert/alt1'],
+        'mixed link types: only alternate extracted',
+    );
+}
+
+# --- RFC 8288: relation types are case-insensitive ---
+{
+    my $resp = MockResponse->new(
+        link => '<https://ca.example/cert/alt1>;rel="Alternate"',
+    );
+    my @urls = Net::ACME2::_parse_link_alternates($resp);
+    is_deeply(
+        \@urls,
+        ['https://ca.example/cert/alt1'],
+        'mixed-case "Alternate" matches (RFC 8288 case-insensitive)',
+    );
+}
+
+{
+    my $resp = MockResponse->new(
+        link => '<https://ca.example/cert/alt1>;rel="ALTERNATE"',
+    );
+    my @urls = Net::ACME2::_parse_link_alternates($resp);
+    is_deeply(
+        \@urls,
+        ['https://ca.example/cert/alt1'],
+        'uppercase "ALTERNATE" matches (RFC 8288 case-insensitive)',
+    );
+}
+
+# --- Whitespace variations in Link header ---
+{
+    my $resp = MockResponse->new(
+        link => '<https://ca.example/cert/alt1> ; rel="alternate"',
+    );
+    my @urls = Net::ACME2::_parse_link_alternates($resp);
+    is_deeply(
+        \@urls,
+        ['https://ca.example/cert/alt1'],
+        'extra whitespace around semicolon',
+    );
+}
+
+done_testing();