pw_transfer: Set resource ID field when session is not known

This updates the transfer server to always set the resource ID in v2
transfers when the session is not known (no handler / no available
context), allowing clients to match against it exclusively when
present.

Change-Id: I1b909df19040fe864c010018ac74f2121b74facd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/118392
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
diff --git a/pw_transfer/transfer_thread_test.cc b/pw_transfer/transfer_thread_test.cc
index 698857c..d25398e 100644
--- a/pw_transfer/transfer_thread_test.cc
+++ b/pw_transfer/transfer_thread_test.cc
@@ -337,6 +337,7 @@
   EXPECT_EQ(id.value(), 7u);
   auto chunk = DecodeChunk(ctx_.response());
   EXPECT_EQ(chunk.session_id(), 7u);
+  EXPECT_EQ(chunk.resource_id(), 7u);
   ASSERT_TRUE(chunk.status().has_value());
   EXPECT_EQ(chunk.status().value(), Status::NotFound());