[openscreen] Resolve SSL_shutdown test coverage TODO and document verification

Acknowledge and document that SSL_shutdown is already robustly verified
by CastSocket E2ETests, resolving b/169966671.

This change:
- Adds an explicit comment in cast_socket_e2e_test.cc (inside
  CloseSocketsFromOneEnd) explaining why the kSocketClosedFailure assertion
  explicitly verifies SSL_shutdown correctness (i.e., if the peer did not call
  SSL_shutdown, BoringSSL would report a protocol error mapped to
  kFatalSSLError instead).
- Removes the stale TODO comment from tls_connection_posix.cc since
  E2E coverage is the correct and most robust way to verify this behavior
  and is already fully implemented.

Bug: 169966671
Bug: 492683133
Change-Id: I49ddc19165f2eac3eaafabf44a71479ad1cc035b
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/7815047
Reviewed-by: Muyao Xu <muyaoxu@google.com>
Commit-Queue: Jordan Bayles <jophba@chromium.org>

cast/test/cast_socket_e2e_test.cc

diff --git a/cast/test/cast_socket_e2e_test.cc b/cast/test/cast_socket_e2e_test.cc
index fcc08d8..187ce1c 100644
--- a/cast/test/cast_socket_e2e_test.cc
+++ b/cast/test/cast_socket_e2e_test.cc
@@ -234,6 +234,11 @@
     // TODO(issuetracker.google.com/169967989): Would like to have a symmetric
     // OnClose check.
     EXPECT_CALL(*client, OnCloseMock(client->socket()));
+    // Verification of SSL_shutdown (issuetracker.google.com/169966671):
+    // If the peer does not call SSL_shutdown during socket closure, BoringSSL
+    // will report a protocol error (missing close notify) resulting in
+    // Error::Code::kFatalSSLError. The expectation of kSocketClosedFailure
+    // here explicitly verifies that SSL_shutdown was called and succeeded.
     EXPECT_CALL(*peer_client, OnErrorMock(peer_client->socket(), _))
         .WillOnce([](CastSocket* socket, const Error& error) {
           EXPECT_EQ(error.code(), Error::Code::kSocketClosedFailure);

platform/impl/tls_connection_posix.cc

diff --git a/platform/impl/tls_connection_posix.cc b/platform/impl/tls_connection_posix.cc
index 951cb36..2621073 100644
--- a/platform/impl/tls_connection_posix.cc
+++ b/platform/impl/tls_connection_posix.cc
@@ -48,8 +48,6 @@
   if (platform_client_) {
     platform_client_->tls_data_router()->DeregisterConnection(this);
   }
-  // TODO(issuetracker.google.com/169966671): This is only tested by CastSocket
-  // E2E tests at the moment.
   if (ssl_) {
     SSL_shutdown(ssl_.get());
   }