[BUG] NPE from null RemoteAddress

- severity: Medium
- files: `shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-grpc/src/main/java/org/apache/shenyu/plugin/grpc/GrpcPlugin.java:99-100`
- description: `Objects.requireNonNull(exchange.getRequest().getRemoteAddress()).getAddress().getHostAddress()` throws NPE if `getRemoteAddress()` returns null (connection closed, proxied without forwarded headers).
- impact: Uncaught NPE fails the gRPC request entirely.
- suggested_fix: Guard with `Optional.ofNullable(...)` and use a fallback.
- confidence: Medium
- related_existing: none

---
_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._

Start in shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-grpc/src/main/java/org/apache/shenyu/plugin/grpc/GrpcPlugin.java at lines 99-100 and trace how the remote address is used. Verify the request path when getRemoteAddress() returns null, including closed or proxied connections, and confirm that it completes without an uncaught NPE by using the agreed fallback.

Written by the indexing model from the issue text.