[modbus_client] Add continuous option to the read and send actions (#18542)

Co-authored-by: Claude <noreply@anthropic.com>

tests/component_tests/modbus_client/test_modbus_client.py

@@ -16,7 +16,13 @@ from esphome.components.modbus_client import (
     CONFIG_SCHEMA,
     MODBUS_CLIENT_SEND_SCHEMA,
 )
-from esphome.const import CONF_ADDRESS, CONF_ID, CONF_ON_ERROR, CONF_ON_RESPONSE
+from esphome.const import (
+    CONF_ADDRESS,
+    CONF_CONTINUOUS,
+    CONF_ID,
+    CONF_ON_ERROR,
+    CONF_ON_RESPONSE,
+)
 from esphome.core import Lambda
 from esphome.types import ConfigType
 
@@ -118,6 +124,29 @@ def test_on_no_response_retry_lambda_accepted() -> None:
     )
 
 
+def test_continuous_on_write_pdu_rejected() -> None:
+    """A literal write-code PDU with continuous: true is rejected at config time (reads only)."""
+    with pytest.raises(cv.Invalid, match="does not apply to a write PDU"):
+        MODBUS_CLIENT_SEND_SCHEMA(
+            {
+                CONF_ADDRESS: 0x01,
+                CONF_PDU: [0x06, 0x00, 0x01, 0x00, 0x0A],
+                CONF_CONTINUOUS: True,
+            }
+        )
+
+
+def test_continuous_on_read_pdu_accepted() -> None:
+    """A literal read-code PDU with continuous: true is fine - continuous polling applies to reads."""
+    MODBUS_CLIENT_SEND_SCHEMA(
+        {
+            CONF_ADDRESS: 0x01,
+            CONF_PDU: [0x03, 0x00, 0x10, 0x00, 0x01],
+            CONF_CONTINUOUS: True,
+        }
+    )
+
+
 # The standalone component block. The compile fixtures cover the accepted shapes end to end; these pin
 # the parts a fixture cannot express - a rejection, and a module flag whose absence breaks other
 # components rather than this one.