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

README.md

@@ -179,10 +179,11 @@ A message has:
   the SimpleX relay, to the second; for a sent one, when the bot sent
   it.
 
-The answer is `400` if the query cannot be read (it holds a `;`, or a
+The answer is `400` if `count` is not a whole number from 1 to 100, and
-`%` not followed by two hexadecimal digits) or `count` is not a whole
+`404` if `GET /api/v1/chats` does not list `id`. A query the server
-number from 1 to 100, and `404` if `GET /api/v1/chats` does not list
+cannot read gets `400` with `the query cannot be read`; examples are a
-`id`.
+query that holds a `;`, a `%` not followed by two hexadecimal digits, or
+more than 10,000 parts separated by `&`.
 
 ### `POST /api/v1/chats/{id}/messages`
 

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),