fix(web): keep exception messages out of API responses (py/stack-trace-exposure)

CodeQL had ~40 open py/stack-trace-exposure alerts on main. Almost all
flowed through describe_exception(), which returned "TypeName: message"
(redacted, capped); the rest through _run_systemctl_command's str(err),
WiFiManager's `return False, str(e)`, unit_refresh's f-strings and two
str(e)/f"{err}" messages in api_v3/__init__.py.

describe_exception() now returns a reason code -- the type, plus the
errno symbol for an OSError ("OSError:EIO", "PermissionError:EACCES") --
and logs the redacted message itself. That keeps what #538 wanted (a
failing disk still says EIO in the response) without quoting paths,
URLs or library internals, and fixes every call site at once; the
test_no_api_v3_handler_discards_its_exception policy still holds.

Service results: _get_display_service_status returns active/returncode
only, and the on-demand start/stop `service` result keeps
returncode/active/started/status but drops systemctl stdout/stderr
(logged on failure). Nothing in web_interface/static, the templates or
the MQTT bridge reads those fields. The Starlark SIGKILL-restart error
no longer returns systemctl stderr as `details`.

WiFi, unit-refresh, config-save and plugin-removal failures now say
what failed with the reason code and point at the log. display.py is
untouched (draft #773 edits it).

Tests: test_api_v3_no_exception_text.py drives one route per affected
file with a marker in the exception message and asserts it never
reaches the body; all 13 fail on origin/main, and targeted mutations
(drop the service filter, put stderr back, str(e) in WiFiManager,
{e} in unit_refresh, {install_err} in system.py, message back in
describe_exception) each fail at least one. Tests that asserted the old
message-in-details contract now assert the reason code.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/web_interface/test_api_v3_unhandled_errors.py

@@ -114,12 +114,11 @@ def test_the_answer_is_what_the_catch_all_returned(client, caplog, method, url,
     assert records[-1].exc_info[1] is FORCED
 
 
-def test_credentials_are_redacted_from_the_detail(client):
+def test_the_exception_message_never_reaches_the_detail(client):
     body = client.get("/api/v3/plugins/installed").get_json()
-    for secret in ("SECRET123", "pw1", "K1"):
-        assert secret not in body["details"]
-    assert "<redacted>" in body["details"]
-    assert body["details"].startswith("RuntimeError: forced failure")
+    for secret in ("SECRET123", "pw1", "K1", "forced failure"):
+        assert secret not in str(body)
+    assert body["details"] == "RuntimeError"
 
 
 def _raise_415():
@@ -218,7 +217,7 @@ class TestPluginActionStep1:
             encoding="utf-8")
         return d
 
-    def test_the_script_error_reaches_the_response(self, plugin_dir, monkeypatch):
+    def test_the_script_error_is_reported_by_type(self, plugin_dir, monkeypatch):
         from unittest.mock import MagicMock
         manager = MagicMock()
         manager.get_plugin_directory.return_value = str(plugin_dir)
@@ -232,5 +231,6 @@ class TestPluginActionStep1:
 
         assert resp.status_code == 500
         body = resp.get_json()
-        assert body["details"] == "RuntimeError: the auth script failed"
+        assert body["details"] == "RuntimeError"
+        assert "the auth script failed" not in str(body)
         assert body["message"] == 'An error occurred; see logs for details'