Add diagnostics platform to DSMR Reader (#115805)

* Add diagnostics platform

* Feedback

---------

Co-authored-by: Joost Lekkerkerker <joostlek@outlook.com>

tests/components/dsmr_reader/test_diagnostics.py

@@ -0,0 +1,39 @@
+"""Test the DSMR Reader component diagnostics."""
+
+from unittest.mock import patch
+
+from syrupy import SnapshotAssertion
+
+from homeassistant.components.dsmr_reader.const import DOMAIN
+from homeassistant.core import HomeAssistant
+
+from tests.common import MockConfigEntry
+from tests.components.diagnostics import get_diagnostics_for_config_entry
+from tests.typing import ClientSessionGenerator
+
+
+async def test_get_config_entry_diagnostics(
+    hass: HomeAssistant,
+    snapshot: SnapshotAssertion,
+    hass_client: ClientSessionGenerator,
+) -> None:
+    """Test if get_config_entry_diagnostics returns the correct data."""
+    config_entry = MockConfigEntry(
+        domain=DOMAIN,
+        title=DOMAIN,
+        options={},
+        entry_id="TEST_ENTRY_ID",
+        unique_id="UNIQUE_TEST_ID",
+    )
+    config_entry.add_to_hass(hass)
+
+    with patch(
+        "homeassistant.components.dsmr_reader.async_setup_entry", return_value=True
+    ):
+        await hass.config_entries.async_setup(config_entry.entry_id)
+        await hass.async_block_till_done()
+
+    diagnostics = await get_diagnostics_for_config_entry(
+        hass, hass_client, config_entry
+    )
+    assert diagnostics == snapshot