fix(mtproto): leave clients mtg cannot parse out of the generated config

One mtg-multi process serves every client of an MTProto inbound from a
single TOML file and refuses the whole file over one value it cannot
parse. InstanceFromInbound already guarded the ad tag for that reason
but copied secret, quota and expiry as stored, so a malformed secret, a
year-10000 expiry or a near-MaxInt64 quota on one client took the
sidecar down for everyone on the inbound.

A client whose secret mtg cannot parse is left out with a warning, and
quota and expiry are capped at what mtg accepts.

internal/database/model/model_mtproto_test.go

@@ -25,6 +25,20 @@ func TestGenerateFakeTLSSecret(t *testing.T) {
 	}
 }
 
+func TestValidMtprotoSecret(t *testing.T) {
+	const good = "ee0123456789abcdef0123456789abcdef6578616d706c652e636f6d"
+	for _, s := range []string{good, strings.ToUpper(good), "ee0123456789abcdef0123456789abcdef61", "7gEjRWeJq83vASNFZ4mrze9leGFtcGxlLmNvbQ"} {
+		if !ValidMtprotoSecret(s) {
+			t.Fatalf("mtg parses %q, so it must be valid", s)
+		}
+	}
+	for _, s := range []string{"", "zz", "ee00", "dd0123456789abcdef0123456789abcdef6578616d706c652e636f6d", "ee0123456789abcdef0123456789abcdef", "7gEjRWeJq83vASNFZ4mrze9leGFtcGxlLmNvbQ==", "not a secret"} {
+		if ValidMtprotoSecret(s) {
+			t.Fatalf("mtg refuses %q, so it must be invalid", s)
+		}
+	}
+}
+
 func TestStripMtprotoInboundAdTag(t *testing.T) {
 	in := `{"adTag":"0123456789abcdef0123456789abcdef","clients":[{"email":"a","adTag":"fedcba9876543210fedcba9876543210"}]}`
 	out, changed := StripMtprotoInboundAdTag(in)