README: state the rule for a query that cannot be read (closes #5)

The README listed the queries that GET /api/v1/chats/{id}/messages
cannot read as if the list were complete, but a query of more than
10,000 parts separated by & is refused the same way, because Go's
url.ParseQuery takes no more. The README now states the rule, that a
query the server cannot read gets 400 with "the query cannot be read",
and gives the three cases as examples. TestMessagesUnreadableQuery
covers the 10,000-part case.

Model: opus-5-5

internal/api/messages_test.go

@@ -152,6 +152,7 @@ func TestMessagesCount(t *testing.T) {
 
 // TestMessagesUnreadableQuery: a query that cannot be decoded is refused
 // with a sentence that says so, whether or not the bad part is count.
+// The last query has 10,001 parts, more than Go's url.ParseQuery takes.
 func TestMessagesUnreadableQuery(t *testing.T) {
 	t.Parallel()
 
@@ -159,6 +160,7 @@ func TestMessagesUnreadableQuery(t *testing.T) {
 
 	for _, query := range []string{
 		"?count=1%", "?count=5&x=%zz", "?x=%zz", "?count=5;x=1",
+		"?count=5" + strings.Repeat("&", 10000),
 	} {
 		client := &fakeClient{contacts: oneChat()}
 		rec := request(t, newAPI(credential, client),