Add encoding tests for API framework

Add tests that check that the API framework is able
to handle various character classes. This also serves
to document current behaviour.

Author: stsnel

tests/features/api/api_ping.feature

@@ -0,0 +1,26 @@
+@api
+Feature: API (de)serialization using api_ping
+
+# These tests use the api_ping endpoint (only available in development environments)
+# to test how strings with different encodings are passed to and/from API endpoints.
+# This is meant to test behaviour of the common API code in util/api.py (in combination
+# with iRODS and PRC).
+
+    Scenario Outline: The object echo endpoint returns <case> unchanged
+        Given user technicaladmin is authenticated
+        When the ping API is queried with the "<case>" payload
+        Then the response status code is "200"
+        And the ping response returns the "<case>" payload unchanged
+
+        Examples:
+            | case                   |
+            | empty_string           |
+            | ascii_only_letters     |
+            | ascii_with_numbers     |
+            | ascii_with_punctuation |
+            | ascii_with_newline     |
+            | ascii_long_10k         |
+	    | non_ascii_letters      |
+	    | control_chars          |
+	    | cjk                    |
+            | emoji_astral           |

tests/step_defs/api/test_api_ping.py

@@ -0,0 +1,55 @@
+# coding=utf-8
+"""Ping/echo API feature tests.
+
+These tests document how the API framework in util/api.py (de)serializes
+various character encodings, end-to-end through the normal API request path
+(python-irodsclient, iRODS and back). 
+"""
+
+__copyright__ = 'Copyright (c) 2026, Utrecht University'
+__license__   = 'GPLv3, see LICENSE'
+
+from pytest_bdd import (
+    parsers,
+    scenarios,
+    then,
+    when,
+)
+
+from conftest import api_request
+
+scenarios('../../features/api/api_ping.feature')
+
+# String payloads echoed back verbatim by api_ping
+STRING_PAYLOADS = {
+    "empty_string": "",
+    "ascii_only_letters": "Hello world",
+    "ascii_with_numbers": "The 39 steps",
+    "ascii_with_punctuation": "Hello `~!@#$%^&*()-_=+;:'\"\\|<>,.?/",
+    "ascii_with_newline": "Line 1\nLine 2",
+    "ascii_long_10k": 9000 * "a",
+    "non_ascii_letters": "blåbær smör mjólk brauð",
+    "control_chars": "tab\there newline\nreturn\rnuli\0bell\a",
+    "cjk": "日本語 한국어",  # Kanji and Korean Unicode
+    "emoji_astral": "\U0001f600\U0001f389", # Unicode outside basic multilingual plane (BMP)
+}
+
+
+@when(parsers.parse('the ping API is queried with the "{case}" payload'), target_fixture="api_response")
+def ping_query(user, case):
+    return api_request(user, "ping", {"x": STRING_PAYLOADS[case]})
+
+
+@then(parsers.parse('the ping response returns the "{case}" payload unchanged'))
+def ping_response_echoes(api_response, case):
+    _, body = api_response
+    payload = STRING_PAYLOADS[case]
+
+    assert body["status"] == "ok", body
+    assert body["data"] == payload
+
+
+@then(parsers.parse('the ping response is an error named "{name}"'))
+def ping_response_error(api_response, name):
+    _, body = api_response
+    assert body["status"] == "error_" + name, body