Implement the birthday wisher core (MIL-002)

Layered package (domain, application, infrastructure, presentation), CLI with
--dry-run and --today, SMTP with STARTTLS behind a port, 56 offline tests and
the Python code review RC-006.

Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5
Task: MIL-002#6
Refs #7
Refs #8
Refs #9
Refs #10
Refs #11
Refs #12

tests/integration/infrastructure/test_settings.py

@@ -0,0 +1,46 @@
+"""Tests for reading the sender credentials from an environment mapping."""
+
+import pytest
+
+from birthday_wisher import constants
+from birthday_wisher.domain.errors import ConfigurationError
+from birthday_wisher.infrastructure.settings import Credentials, read_credentials
+
+ADDRESS = constants.ENV_SENDER_ADDRESS
+TOKEN = constants.ENV_SENDER_TOKEN
+
+
+def test_read_credentials_returns_address_and_token() -> None:
+    """Both variables present give the credentials."""
+    result = read_credentials({ADDRESS: "me@example.com", TOKEN: "tok"})
+    assert result == Credentials("me@example.com", "tok")
+
+
+def test_read_credentials_reports_missing_names_not_values() -> None:
+    """The error names the missing variable and shows no value."""
+    with pytest.raises(ConfigurationError) as info:
+        read_credentials({ADDRESS: "me@example.com"})
+    text = str(info.value)
+    assert TOKEN in text
+    assert "me@example.com" not in text
+
+
+def test_read_credentials_reports_both_when_none_set() -> None:
+    """Both missing names are listed."""
+    with pytest.raises(ConfigurationError) as info:
+        read_credentials({})
+    assert ADDRESS in str(info.value)
+    assert TOKEN in str(info.value)
+
+
+def test_read_credentials_treats_empty_value_as_missing() -> None:
+    """An empty value counts as missing."""
+    with pytest.raises(ConfigurationError, match=TOKEN):
+        read_credentials({ADDRESS: "me@example.com", TOKEN: ""})
+
+
+def test_credentials_repr_hides_token() -> None:
+    """Neither repr nor str shows the token."""
+    credentials = Credentials("me@example.com", "super-secret")
+    assert "super-secret" not in repr(credentials)
+    assert "super-secret" not in str(credentials)