Read a trusted_proxies entry in the IPv4-mapped form as its IPv4 range (closes #303)

An entry such as ::ffff:10.0.0.0/104 in trusted_proxies was accepted
but trusted nothing, because the client address is matched as the IPv4
address an IPv4-mapped address holds. The conversion that blocked_networks
gained in #301 now lives in
parseCIDRList, which reads both settings: such an entry is read as the
IPv4 range it covers, and one shorter than /96 stops startup with an
error naming the setting being read and quoting the entry as written.
README.md says so for trusted_proxies.

Model: opus-5-5

internal/config/trusted_proxies_internal_test.go

@@ -83,3 +83,41 @@ func TestTrustedProxiesInvalidAbortsStartup(t *testing.T) {
 		},
 	})
 }
+
+// TestTrustedProxiesMappedEntryIsIPv4Range checks that an entry written in
+// the IPv4-mapped form (::ffff:a.b.c.d) is kept as the IPv4 range it covers,
+// since the client address is matched as the IPv4 address it holds. /96, the
+// shortest such entry, covers every IPv4 address.
+func TestTrustedProxiesMappedEntryIsIPv4Range(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine+
+		`trusted_proxies: ["::ffff:10.0.0.0/104", "::ffff:0:0/96"]`+"\n")
+	if err != nil {
+		t.Fatalf("IPv4-mapped trusted_proxies entries should load: %v", err)
+	}
+
+	got := make([]string, len(c.TrustedProxies))
+	for i, p := range c.TrustedProxies {
+		got[i] = p.String()
+	}
+
+	if joined := strings.Join(got, ","); joined != "10.0.0.0/8,0.0.0.0/0" {
+		t.Errorf("TrustedProxies = %v, want 10.0.0.0/8 and 0.0.0.0/0", got)
+	}
+}
+
+// TestTrustedProxiesMappedEntryShorterThan96AbortsStartup checks that an
+// entry in the IPv4-mapped form shorter than /96, which covers more than the
+// IPv4 addresses, aborts startup with an error naming the key and the value.
+func TestTrustedProxiesMappedEntryShorterThan96AbortsStartup(t *testing.T) {
+	t.Parallel()
+
+	runAbortCases(t, []abortCase{
+		{
+			name:              "mapped-95",
+			yaml:              signingKeyLine + `trusted_proxies: ["::ffff:0:0/95"]` + "\n",
+			wantErrSubstrings: []string{keyTrustedProxies, "::ffff:0:0/95"},
+		},
+	})
+}