[posix] implement otPlatTcp platform abstraction (openthread#13193)

This commit provides a complete POSIX implementation of the new
`otPlatTcp` platform abstraction APIs. It leverages the existing
OpenThread `Mainloop` polling mechanism to build a fully event-driven,
non-blocking TCP socket implementation without requiring extra threads.

Key features and implementation details:
- Uses `Mainloop` read/write/error sets to monitor socket states.
- Implements `otPlatTcpEnableListener` and `otPlatTcpAccept` to handle
  incoming connection requests over IPv6.
- Implements `otPlatTcpConnect` for asynchronous non-blocking outgoing
  TCP connections, monitoring `SO_ERROR` to detect handshake completion.
- Handles data transmission utilizing `otPlatTcpIsTxPending` to
  monitor the write descriptor and correctly signal `HandleTxReady`.
- Implements `otPlatTcpSend` and `otPlatTcpReceive` ensuring proper
  buffering, suppressing `SIGPIPE` safely using `SO_NOSIGPIPE` and
  `MSG_NOSIGNAL`.
- Encodes socket file descriptors directly within the platform data
  `mData.mDescriptor` union, avoiding dynamic memory allocation.

Author: abtink

src/posix/platform/CMakeLists.txt

@@ -152,6 +152,7 @@ add_library(openthread-posix
     spinel_manager.cpp
     spi_interface.cpp
     system.cpp
+    tcp.cpp
     tmp_storage.cpp
     trel.cpp
     udp.cpp

src/posix/platform/system.cpp

@@ -60,6 +60,7 @@
 #include "posix/platform/radio_url.hpp"
 #include "posix/platform/spinel_driver_getter.hpp"
 #include "posix/platform/spinel_manager.hpp"
+#include "posix/platform/tcp.hpp"
 #include "posix/platform/udp.hpp"
 
 otInstance *gInstance = nullptr;
@@ -149,6 +150,10 @@ void platformInitRcpMode(otPlatformConfig *aPlatformConfig)
     ot::Posix::InfraNetif::Get().Init();
 #endif
 
+#if OPENTHREAD_CONFIG_PLATFORM_TCP_ENABLE
+    ot::Posix::Tcp::Get().Init();
+#endif
+
 #if OPENTHREAD_CONFIG_MULTICAST_DNS_ENABLE
     ot::Posix::MdnsSocket::Get().Init();
 #endif
@@ -169,6 +174,7 @@ void platformInitRcpMode(otPlatformConfig *aPlatformConfig)
     ot::Posix::Udp::Get().Init(aPlatformConfig->mInterfaceName);
 #endif
 #endif
+
 exit:
     return;
 }
@@ -248,6 +254,10 @@ void platformSetUp(otPlatformConfig *aPlatformConfig)
     ot::Posix::Udp::Get().SetUp();
 #endif
 
+#if OPENTHREAD_CONFIG_PLATFORM_TCP_ENABLE
+    ot::Posix::Tcp::Get().SetUp();
+#endif
+
 #if OPENTHREAD_CONFIG_MULTICAST_DNS_ENABLE
     ot::Posix::MdnsSocket::Get().SetUp();
 #endif
@@ -310,6 +320,10 @@ void platformTearDown(void)
     ot::Posix::Udp::Get().TearDown();
 #endif
 
+#if OPENTHREAD_CONFIG_PLATFORM_TCP_ENABLE
+    ot::Posix::Tcp::Get().TearDown();
+#endif
+
 #if OPENTHREAD_CONFIG_PLATFORM_NETIF_ENABLE
     platformNetifTearDown();
 #endif
@@ -341,6 +355,11 @@ void platformDeinitRcpMode(void)
 #if OPENTHREAD_CONFIG_PLATFORM_UDP_ENABLE
     ot::Posix::Udp::Get().Deinit();
 #endif
+
+#if OPENTHREAD_CONFIG_PLATFORM_TCP_ENABLE
+    ot::Posix::Tcp::Get().Deinit();
+#endif
+
 #if OPENTHREAD_CONFIG_PLATFORM_NETIF_ENABLE
     platformNetifDeinit();
 #endif