feat: implement inbound XMUX form fields (#5211)

* feat: implement inbound XMUX form fields

* fix: replace any cast to satisfy eslint

* test: update xhttp form snapshot for XMUX

* fix(inbound): persist xmux on save so the XMUX form actually round-trips

The inbound wire normalizer unconditionally deleted xhttpSettings.xmux,
so the new inbound XMUX form was stripped on save and never reached the
stored config — the subscription extra blob (buildXhttpExtra) could
never see it. Gate the deletion on the enableXmux toggle, mirroring the
outbound adapter, and add regression tests for both on/off cases.

* fix(xmux): enforce xray-core's maxConnections/maxConcurrency exclusivity

xray-core's XmuxConfig rejects a config that sets both maxConnections
and maxConcurrency. The panel pre-fills maxConcurrency ('16-32') whenever
XMUX is enabled, so an explicit maxConnections would always collide and
make xray refuse the config. Mirror core's semantics in the wire
normalizer: when maxConnections is set (>0, an explicit opt-in since it
defaults to 0), drop the leftover default maxConcurrency. Applies to both
inbound and outbound xhttp.

---------

Co-authored-by: Sanaei <ho3ein.sanaei@gmail.com>

frontend/src/test/__snapshots__/inbound-form-blocks.test.tsx.snap

@@ -130,5 +130,6 @@ exports[`inbound transport forms > XhttpForm field structure is stable 1`] = `
   "Session Placement",
   "Sequence Placement",
   "No SSE Header",
+  "XMUX",
 ]
 `;