ci: fix Rust setup action in GitHub workflow

- Replaced nonexistent `rust-lang/setup-rust` with `actions-rs/toolchain@v1`
- Ensures stable Rust toolchain, Clippy, and rustfmt are installed in CI
- Temporary config.toml created so unit tests and doctests pass
- CI runs on push to any branch and PRs targeting main/dev

Author: CHINMAYVIVEK

.github/workflows/rust-ci.yml

@@ -25,7 +25,7 @@ jobs:
 
       # 2️⃣ Set up Rust toolchain using latest supported action
       - name: Set up Rust
-        uses: rust-lang/setup-rust@v2
+        uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
           profile: minimal