test(xmtp_mls): run concurrent DM-invite streaming on native d14n

`test_many_concurrent_dm_invites` was ignored on all d14n via
`#[cfg_attr(all(feature = "d14n"), ignore)]` with no rationale — but it passes on
the native d14n backend (5- and 100-concurrent-DM cases; verified twice, the
100-DM case ~30s against its 120s budget). The ignore was over-broad: only
d14n+wasm needs skipping (100 concurrent streaming clients are impractical
there), matching the heavy group tests' `all(feature = "d14n", target_arch =
"wasm32")` gating. Narrow it so native d14n gains the concurrent-streaming
coverage.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017AVuJ7VAgen2fFhtJQ6zid

Author: tylerhawkes

crates/xmtp_mls/src/subscriptions/stream_conversations.rs

@@ -649,7 +649,9 @@ mod test {
     #[case::onehundred_dms(100)]
     #[xmtp_common::test]
     #[awt]
-    #[cfg_attr(all(feature = "d14n"), ignore)]
+    // Runs on native d14n (verified); still skipped on d14n+wasm, where 100
+    // concurrent streaming clients are impractical (matches the heavy group tests).
+    #[cfg_attr(all(feature = "d14n", target_arch = "wasm32"), ignore)]
     async fn test_many_concurrent_dm_invites(#[future] alix: ClientTester, #[case] dms: usize) {
         let alix_inbox_id = Arc::new(alix.inbox_id().to_string());
         let mut clients = vec![];