test: |Worker| add junk_mail_policy regression tests for issue #1084 (#1089)

Cover the junk-mail policy behavior fixed in #1085:
- none/neutral results for SPF/DKIM/DMARC are treated as the method
  being absent and do not trigger JUNK_MAIL_CHECK_LIST rejection
- explicit fail results are still rejected
- JUNK_MAIL_FORCE_PASS_LIST only accepts an explicit pass

Run with: node --test-isolation=none --test worker/src/email/junk_mail_policy.test.mjs

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

worker/src/email/junk_mail_policy.test.mjs

@@ -0,0 +1,81 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { isJunkMailByHeaders } from "./junk_mail_policy.ts";
+
+const authenticationResultsHeader = (results) => [{
+    key: "Authentication-Results",
+    value: `mx.example; ${results}`,
+}];
+
+test("issue #1084: dmarc=none does not reject an otherwise passing message", () => {
+    const headers = authenticationResultsHeader(
+        "spf=pass smtp.mailfrom=sender.example; " +
+        "dkim=pass header.d=sender.example; " +
+        "dmarc=none; arc=pass"
+    );
+
+    assert.equal(
+        isJunkMailByHeaders(
+            headers,
+            ["spf", "dkim", "dmarc"],
+            ["spf"],
+        ),
+        false,
+    );
+});
+
+for (const [method, result] of [
+    ["spf", "none"],
+    ["spf", "neutral"],
+    ["dkim", "none"],
+    ["dkim", "neutral"],
+    ["dmarc", "none"],
+    ["dmarc", "neutral"],
+]) {
+    test(`${method}=${result} is absent for JUNK_MAIL_CHECK_LIST`, () => {
+        assert.equal(
+            isJunkMailByHeaders(
+                authenticationResultsHeader(`${method}=${result}`),
+                [method],
+                [],
+            ),
+            false,
+        );
+    });
+}
+
+for (const method of ["spf", "dkim", "dmarc"]) {
+    test(`${method}=fail is junk when the method is checked`, () => {
+        assert.equal(
+            isJunkMailByHeaders(
+                authenticationResultsHeader(`${method}=fail`),
+                [method],
+                [],
+            ),
+            true,
+        );
+    });
+}
+
+test("JUNK_MAIL_FORCE_PASS_LIST requires an explicit pass result", () => {
+    for (const result of ["none", "neutral", "fail"]) {
+        assert.equal(
+            isJunkMailByHeaders(
+                authenticationResultsHeader(`spf=${result}`),
+                [],
+                ["spf"],
+            ),
+            true,
+        );
+    }
+
+    assert.equal(
+        isJunkMailByHeaders(
+            authenticationResultsHeader("spf=pass"),
+            [],
+            ["spf"],
+        ),
+        false,
+    );
+});