[TEST] Say that the overlapping callbacks are a record, not a contract

The case asserts that two callbacks are inside the handler at once, which
is what this client does today. EventHandler does not say whether one
request's callbacks can overlap, so a handler written against the
interface is not obliged to be re-entrant, and an assertion that reads
like a promise is the wrong thing to leave behind.

The number stays, because reaching that overlap is the point of the case
and a client that began serialising callbacks per operation should be
noticed rather than absorbed. The comment now says which of the two it
is.

32 tests pass.

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>

Author: thc1006

ext/test/http/curl_http_test.cc

@@ -942,6 +942,12 @@ TEST_F(BasicCurlHttpTests, CancelFromConnectingWhilePollingCompletes)
   session->FinishSession();
   session_manager->FinishAllSessions();
 
+  // Two callbacks are in the handler at once here. That is what this client does today, not
+  // something EventHandler promises: the interface says nothing about whether one request's
+  // callbacks can overlap, so a handler written against it is not obliged to be re-entrant. The
+  // number is pinned because this case exists to reach that overlap, and a client that started
+  // serialising callbacks per operation would be an improvement worth noticing rather than a
+  // silent change. Read it as a record of the shape, not as a contract to preserve.
   EXPECT_EQ(2, handler->max_concurrent_events_.load(std::memory_order_acquire));
   EXPECT_FALSE(handler->got_response_.load(std::memory_order_acquire));
   EXPECT_EQ(1, handler->cancelled_from_callback_.load(std::memory_order_acquire));