Upgrade rmcp to 0.8.4 (#6234)

Picks up https://github.com/modelcontextprotocol/rust-sdk/pull/509 which
fixes https://github.com/openai/codex/issues/6164

codex-rs/Cargo.lock

@@ -5010,9 +5010,9 @@ dependencies = [
 
 [[package]]
 name = "rmcp"
-version = "0.8.3"
+version = "0.8.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1fdad1258f7259fdc0f2dfc266939c82c3b5d1fd72bcde274d600cdc27e60243"
+checksum = "a2c66318b30535ccd0d3b39afaa4240d6b5a35328fb7672a28e3386c97472805"
 dependencies = [
  "base64",
  "bytes",
@@ -5044,9 +5044,9 @@ dependencies = [
 
 [[package]]
 name = "rmcp-macros"
-version = "0.8.3"
+version = "0.8.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ede0589a208cc7ce81d1be68aa7e74b917fcd03c81528408bab0457e187dcd9b"
+checksum = "49a19193e0d69bb1c96324b1b4daec078d4c8e76d734e53404c107437858a4d2"
 dependencies = [
  "darling 0.21.3",
  "proc-macro2",