oblivious_http:Update comments to indicate that Client & Gateway are thread-safe.

Additional Context : Adding this as per @ericorth's suggestions on cl/479424618 comments section (Apologies for not linking the actual comment here, as Critique is not giving us a link to that comment)

PiperOrigin-RevId: 481960460
diff --git a/build/source_list.bzl b/build/source_list.bzl
index 110f0da..afc80c7 100644
--- a/build/source_list.bzl
+++ b/build/source_list.bzl
@@ -1093,6 +1093,12 @@
     "http2/test_tools/http2_frame_builder_test.cc",
     "http2/test_tools/http2_random_test.cc",
     "http2/test_tools/random_decoder_test_base_test.cc",
+    "oblivious_http/buffers/oblivious_http_integration_test.cc",
+    "oblivious_http/buffers/oblivious_http_request_test.cc",
+    "oblivious_http/buffers/oblivious_http_response_test.cc",
+    "oblivious_http/common/oblivious_http_header_key_config_test.cc",
+    "oblivious_http/oblivious_http_client_test.cc",
+    "oblivious_http/oblivious_http_gateway_test.cc",
     "quic/core/congestion_control/bandwidth_sampler_test.cc",
     "quic/core/congestion_control/bbr2_simulator_test.cc",
     "quic/core/congestion_control/bbr_sender_test.cc",
@@ -1438,6 +1444,20 @@
 binary_http_srcs = [
     "binary_http/binary_http_message.cc",
 ]
+oblivious_http_hdrs = [
+    "oblivious_http/buffers/oblivious_http_request.h",
+    "oblivious_http/buffers/oblivious_http_response.h",
+    "oblivious_http/common/oblivious_http_header_key_config.h",
+    "oblivious_http/oblivious_http_client.h",
+    "oblivious_http/oblivious_http_gateway.h",
+]
+oblivious_http_srcs = [
+    "oblivious_http/buffers/oblivious_http_request.cc",
+    "oblivious_http/buffers/oblivious_http_response.cc",
+    "oblivious_http/common/oblivious_http_header_key_config.cc",
+    "oblivious_http/oblivious_http_client.cc",
+    "oblivious_http/oblivious_http_gateway.cc",
+]
 qbone_hdrs = [
     "quic/qbone/bonnet/icmp_reachable.h",
     "quic/qbone/bonnet/icmp_reachable_interface.h",