Add tests for register_component_source, remove redundant index check

- Test empty name returns 0
- Test deduplication returns same index
- Test overflow warns and returns 0
- Remove duplicate index==0 check in get_component_log_str() since
  component_source_lookup() already handles it

tests/unit_tests/test_cpp_helpers.py

@@ -1,8 +1,10 @@
+import logging
 from unittest.mock import Mock
 
 import pytest
 
 from esphome import const, cpp_helpers as ch
+from esphome.cpp_helpers import ComponentSourcePool, register_component_source
 
 
 @pytest.mark.asyncio
@@ -78,3 +80,32 @@ async def test_register_component__with_setup_priority(monkeypatch):
     assert add_mock.call_count == 4
     app_mock.register_component_.assert_called_with(var)
     assert core_mock.component_ids == []
+
+
+def test_register_component_source_empty_name(monkeypatch):
+    monkeypatch.setattr(ch, "CORE", Mock(data={}))
+    assert register_component_source("") == 0
+
+
+def test_register_component_source_deduplicates(monkeypatch):
+    monkeypatch.setattr(ch, "CORE", Mock(data={}))
+    idx1 = register_component_source("wifi")
+    idx2 = register_component_source("api")
+    idx3 = register_component_source("wifi")
+    assert idx1 == 1
+    assert idx2 == 2
+    assert idx3 == 1  # deduplicated
+
+
+def test_register_component_source_overflow_warns(monkeypatch, caplog):
+    # Pre-fill pool to max
+    pool = ComponentSourcePool(
+        sources={f"comp_{i}": i + 1 for i in range(0xFF)},
+        table_registered=True,
+    )
+    monkeypatch.setattr(ch, "CORE", Mock(data={ch._COMPONENT_SOURCE_DOMAIN: pool}))
+    with caplog.at_level(logging.WARNING):
+        idx = register_component_source("overflow_component")
+    assert idx == 0
+    assert "Too many unique component source names" in caplog.text
+    assert "overflow_component" in caplog.text