Wire realtime WebRTC native media into Bazel (#17145)

- Builds codex-realtime-webrtc through the normal Bazel Rust macro so
native macOS WebRTC sources are included.\n- Shares the macOS -ObjC link
flag with Bazel targets that can link libwebrtc.

---------

Co-authored-by: Codex <noreply@openai.com>

defs.bzl

@@ -31,6 +31,18 @@ WINDOWS_RUSTC_LINK_FLAGS = select({
     "//conditions:default": [],
 })
 
+# libwebrtc uses Objective-C categories from native archives. Any Bazel-linked
+# macOS binary/test that can pull it in must keep category symbols alive.
+MACOS_WEBRTC_RUSTC_LINK_FLAGS = select({
+    "@platforms//os:macos": [
+        "-C",
+        "link-arg=-ObjC",
+        "-C",
+        "link-arg=-lc++",
+    ],
+    "//conditions:default": [],
+})
+
 def multiplatform_binaries(name, platforms = PLATFORMS):
     for platform in platforms:
         platform_data(