fix: Add build_rust/** to workflow paths filter

The codegen-rust job checks for changes to build_rust/src/main.rs
to decide whether to regenerate Rust bindings. Without build_rust/**
in the workflow-level paths filter, changes to the build script alone
would not trigger the workflow, preventing Rust codegen verification.

Applied via @cursor push command

Author: cursoragent

.github/workflows/ci.yml

@@ -7,6 +7,7 @@ on:
       - 'buf.yaml'
       - 'buf.lock'
       - 'buf.gen.yaml'
+      - 'build_rust/**'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}