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.

CMakeLists.txt

@@ -559,6 +559,11 @@ if(BUILD_TESTING)
         ${QT}::Widgets ${QT}::Concurrent ${QT}::Test)
         add_test(NAME test_scanner_ui COMMAND test_scanner_ui)
 
+        add_executable(test_mcp tests/test_mcp.cpp)
+        target_include_directories(test_mcp PRIVATE src)
+        target_link_libraries(test_mcp PRIVATE ${QT}::Core ${QT}::Network ${QT}::Test)
+        add_test(NAME test_mcp COMMAND test_mcp)
+
         if(WIN32)
             add_executable(test_windbg_provider tests/test_windbg_provider.cpp
             plugins/WinDbgMemory/WinDbgMemoryPlugin.cpp