MCP bridge: support multiple concurrent clients

Replace single-client model (m_client/m_readBuffer/m_initialized)
with a ClientState vector. Each client gets its own read buffer and
initialized flag. Responses route to m_currentSender (set during
request processing); notifications broadcast to all initialized
clients.

Re-entrancy guard in onReadyRead: re-resolve ClientState after each
processLine() call since sendJson flush can re-enter the event loop
and trigger onDisconnected, removing the client mid-iteration.

Tests: 378-line test_mcp exercising connect, initialize, tools/list,
disconnect one client, notification broadcast, and serial requests
against a MockMcpServer with the same multi-client architecture.

tests/test_scanner.cpp

@@ -1507,7 +1507,7 @@ private slots:
         data[0x8100] = char(0xFF);
         QVector<MemoryRegion> regions;
         regions.append({0x8000, 0x1000, true, true, false, {}});
-        auto prov = std::make_shared<RegionProvider>(data2, regions);
+        auto prov = std::make_shared<RegionProvider>(data, regions);
         ScanEngine engine;
         QSignalSpy finSpy(&engine, &ScanEngine::finished);
         ScanRequest req;