MAESTRO: add engine TDD fixtures and tests

backend/tests/engine/test_parser.py

@@ -0,0 +1,52 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from app.engine.parser import HeaderParser
+
+
+FIXTURES_DIR = Path(__file__).resolve().parents[1] / "fixtures"
+
+
+@pytest.fixture()
+def sample_headers() -> str:
+    return (FIXTURES_DIR / "sample_headers.txt").read_text(encoding="utf-8")
+
+
+def test_parser_extracts_headers_and_preserves_order(sample_headers: str) -> None:
+    parser = HeaderParser()
+    headers = parser.parse(sample_headers)
+
+    assert headers, "Expected parsed headers to be non-empty."
+
+    indices = [header.index for header in headers]
+    assert indices == list(range(len(headers)))
+
+    names = [header.name for header in headers]
+    assert names[:2] == ["Received", "Received"]
+    assert "X-Should-Not-Be-Parsed" not in names
+
+
+def test_parser_handles_folded_lines(sample_headers: str) -> None:
+    parser = HeaderParser()
+    headers = parser.parse(sample_headers)
+
+    subject = next(header for header in headers if header.name == "Subject")
+    assert "folded line" in subject.value
+    assert "\n" in subject.value
+
+    authentication = next(
+        header for header in headers if header.name == "Authentication-Results"
+    )
+    assert "spf=pass" in authentication.value
+    assert "\n" in authentication.value
+
+
+def test_parser_preserves_content_type_boundary(sample_headers: str) -> None:
+    parser = HeaderParser()
+    headers = parser.parse(sample_headers)
+
+    content_type = next(header for header in headers if header.name == "Content-Type")
+    assert "boundary=\"boundary-123\"" in content_type.value