fix(test-servers): describe and test after-first's resource-subscription counting

Copilot review on #2176. All three findings were the same gap seen from
three angles: `"after-first"` counts *resource-subscription* listens, not
listens, and both the README and the option's own doc comment said
"the first listen".

That distinction is load-bearing rather than incidental — the Inspector
opens a listen at connect time whenever a list-change opt-in is live
(#1920), so counting listens would spend the allowance there and refuse
the user's very first Subscribe, leaving the documented repro describing
a trace nobody sees. It is also what a reader of the option would
otherwise get wrong.

- README: say "the first listen that subscribes to a resource" in both
  places, and name the connect-time listen as the reason.
- `ServerConfig.modern.neverAcknowledgeSubscriptions`: document the
  list-change exemption and contrast it with `true`.
- Integration test for the mode itself, with every list-change opt-in on
  so the connect-time listen really is opened: it must be acknowledged
  without consuming the allowance, the first resource subscription
  acknowledged, the next refused with the stream left `active`.

Closes #2097

Signed-off-by: cliffhall <cliff@futurescale.com>

test-servers/src/composable-test-server.ts

@@ -707,10 +707,21 @@ export interface ServerConfig {
      * same breath". Used by the `subscriptions-never-acknowledged-http`
      * showcase; a conformant server never does this on an opening listen.
      *
-     * `"after-first"` acknowledges the first listen and refuses every one after
-     * it — the *reconnect* shape, and the only way to reach the Inspector's
+     * `"after-first"` acknowledges the first listen that **subscribes to a
+     * resource** and refuses every resource-subscription listen after it — the
+     * *reconnect* shape, and the only way to reach the Inspector's
      * never-acknowledged badge by hand (it is gated on a live subscription,
      * which a server refusing from the outset never lets you hold).
+     *
+     * A listen carrying no `resourceSubscriptions` is always acknowledged under
+     * this mode and does not consume the allowance. That exemption is what makes
+     * the mode reproducible: the Inspector already opens a listen at connect
+     * time when a list-change opt-in is live (#1920), so counting listens rather
+     * than resource-subscription listens would spend the allowance before the
+     * user clicks anything, refusing the very first Subscribe.
+     *
+     * `true` refuses every listen unconditionally, list-change-only ones
+     * included — the literal shape in the report.
      */
     neverAcknowledgeSubscriptions?: boolean | "after-first";
   };