fix(test): use platform-specific path separators in attachment_ref tests

On Windows, paths serialized in attachment-ref JSON payloads contain
backslashes. The strstr assertions were hardcoded with forward slashes,
causing attachment_ref_copy, attachment_ref_move, and attachment_ref_restore
to fail on Windows.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jpnurmi

tests/unit/test_envelopes.c

@@ -1002,8 +1002,13 @@ SENTRY_TEST(attachment_ref_copy)
     payload_json = sentry__value_from_json(payload, payload_len);
     ref_path
         = sentry_value_as_string(sentry_value_get_by_key(payload_json, "path"));
+#ifdef SENTRY_PLATFORM_WINDOWS
+    TEST_CHECK(!!strstr(
+        ref_path, "\\attachments\\c993afb6-b4ac-48a6-b61b-2558e601d65d\\"));
+#else
     TEST_CHECK(!!strstr(
         ref_path, "/attachments/c993afb6-b4ac-48a6-b61b-2558e601d65d/"));
+#endif
     TEST_CHECK_STRING_EQUAL(sentry_value_as_string(sentry_value_get_by_key(
                                 payload_json, "content_type")),
         "application/x-dmp");
@@ -1081,9 +1086,15 @@ SENTRY_TEST(attachment_ref_move)
     payload_json = sentry__value_from_json(payload, payload_len);
     ref_path
         = sentry_value_as_string(sentry_value_get_by_key(payload_json, "path"));
+#ifdef SENTRY_PLATFORM_WINDOWS
+    TEST_CHECK(!!strstr(ref_path,
+        "\\attachments\\c993afb6-b4ac-48a6-b61b-2558e601d65d\\"
+        "test_minidump.dmp"));
+#else
     TEST_CHECK(!!strstr(ref_path,
         "/attachments/c993afb6-b4ac-48a6-b61b-2558e601d65d/"
         "test_minidump.dmp"));
+#endif
 
     sentry_path_t *dest_path = sentry__path_from_str(ref_path);
     TEST_CHECK(sentry__path_is_file(dest_path));
@@ -1119,9 +1130,15 @@ send_restore_envelope(sentry_envelope_t *envelope, void *data)
         sentry_value_t pj = sentry__value_from_json(payload, payload_len);
         const char *ref_path
             = sentry_value_as_string(sentry_value_get_by_key(pj, "path"));
+#ifdef SENTRY_PLATFORM_WINDOWS
+        TEST_CHECK(!!strstr(ref_path,
+            "\\attachments\\c993afb6-b4ac-48a6-b61b-2558e601d65d\\"
+            "test_minidump.dmp"));
+#else
         TEST_CHECK(!!strstr(ref_path,
             "/attachments/c993afb6-b4ac-48a6-b61b-2558e601d65d/"
             "test_minidump.dmp"));
+#endif
         sentry_value_decref(pj);
     }