[MCP] Update rmcp to 0.8.3 (#5542)

Picks up modelcontextprotocol/rust-sdk#497 which fixes #5208 by allowing 204 response to MCP initialize notifications instead of just 202.

codex-rs/Cargo.lock

@@ -4954,9 +4954,9 @@ dependencies = [
 
 [[package]]
 name = "rmcp"
-version = "0.8.2"
+version = "0.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4e35d31f89beb59c83bc31363426da25b323ce0c2e5b53c7bf29867d16ee7898"
+checksum = "1fdad1258f7259fdc0f2dfc266939c82c3b5d1fd72bcde274d600cdc27e60243"
 dependencies = [
  "base64",
  "bytes",
@@ -4988,9 +4988,9 @@ dependencies = [
 
 [[package]]
 name = "rmcp-macros"
-version = "0.8.2"
+version = "0.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d88518b38110c439a03f0f4eee40e5105d648a530711cb87f98991e3f324a664"
+checksum = "ede0589a208cc7ce81d1be68aa7e74b917fcd03c81528408bab0457e187dcd9b"
 dependencies = [
  "darling 0.21.3",
  "proc-macro2",