Guard TCP API writes against dead sockets

Agent-Logs-Url: https://github.com/meshtastic/firmware/sessions/ebeb38d5-7339-4eac-b310-4b6dd9d40758

Co-authored-by: thebentern <9000580+thebentern@users.noreply.github.com>

test/test_client_write_checks/test_main.cpp

@@ -0,0 +1,80 @@
+#include "TestUtil.h"
+#include "mesh/api/ClientWriteChecks.h"
+#include <unity.h>
+
+namespace
+{
+class ClientWithAvailable
+{
+  public:
+    bool connected() const { return connected_; }
+    int availableForWrite() const { return writable_; }
+
+    bool connected_ = true;
+    int writable_ = 64;
+};
+
+class ClientWithoutAvailable
+{
+  public:
+    bool connected() const { return connected_; }
+
+    bool connected_ = true;
+};
+} // namespace
+
+void setUp(void) {}
+void tearDown(void) {}
+
+void test_availableForWrite_is_detected_when_present()
+{
+    TEST_ASSERT_TRUE((stream_api::HasAvailableForWrite<ClientWithAvailable>::value));
+    TEST_ASSERT_FALSE((stream_api::HasAvailableForWrite<ClientWithoutAvailable>::value));
+}
+
+void test_availableForWriteOrUnknown_returns_client_capacity()
+{
+    ClientWithAvailable client;
+    TEST_ASSERT_EQUAL_INT(64, stream_api::availableForWriteOrUnknown(client));
+}
+
+void test_availableForWriteOrUnknown_returns_unknown_when_absent()
+{
+    ClientWithoutAvailable client;
+    TEST_ASSERT_EQUAL_INT(-1, stream_api::availableForWriteOrUnknown(client));
+}
+
+void test_clientReadyForWrite_rejects_disconnected_clients()
+{
+    ClientWithAvailable client;
+    client.connected_ = false;
+    TEST_ASSERT_FALSE(stream_api::clientReadyForWrite(client));
+}
+
+void test_clientReadyForWrite_rejects_non_writable_clients()
+{
+    ClientWithAvailable client;
+    client.writable_ = 0;
+    TEST_ASSERT_FALSE(stream_api::clientReadyForWrite(client));
+}
+
+void test_clientReadyForWrite_allows_clients_without_availableForWrite()
+{
+    ClientWithoutAvailable client;
+    TEST_ASSERT_TRUE(stream_api::clientReadyForWrite(client));
+}
+
+void setup()
+{
+    initializeTestEnvironment();
+    UNITY_BEGIN();
+    RUN_TEST(test_availableForWrite_is_detected_when_present);
+    RUN_TEST(test_availableForWriteOrUnknown_returns_client_capacity);
+    RUN_TEST(test_availableForWriteOrUnknown_returns_unknown_when_absent);
+    RUN_TEST(test_clientReadyForWrite_rejects_disconnected_clients);
+    RUN_TEST(test_clientReadyForWrite_rejects_non_writable_clients);
+    RUN_TEST(test_clientReadyForWrite_allows_clients_without_availableForWrite);
+    exit(UNITY_END());
+}
+
+void loop() {}