Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

The sample is a simple echo driver — it reads a baudrate from device tree (defaulting to 115200), configures the port, and echoes back received data.

```rust
+    fn receive<'bound>(
+        sdev: &'bound serdev::Device<Bound>,
+        _this: Pin<&Self>,
+        data: &[u8],
+    ) -> usize {
+        let _ = sdev.write_all(data, serdev::Timeout::Max);
+        data.len()
+    }
```

The `let _ = ...` explicitly discards the write result, which is acceptable for a sample. The function always returns `data.len()` claiming all data was consumed regardless of write success — fine for demonstration purposes.

**Nit:** The `set_baudrate` error handling:
```rust
+        if sdev
+            .set_baudrate(
+                dev.fwnode()
+                    .and_then(|fwnode| fwnode.property_read(c"baudrate").optional())
+                    .unwrap_or(115200),
+            )
+            .is_err()
+        {
+            return Err(EINVAL);
```
The `Err(u32)` from `set_baudrate` (containing the actual baudrate that was set) is discarded. A `dev_warn!` logging the mismatch would be helpful for debugging, but not required for a sample.

The `// ` trailing comments on some import lines are unusual formatting artifacts (likely from rustfmt vertical formatting). Not a functional issue.

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