[BUG] GrpcPlugin blocks reactor netty event loop on synchronous gRPC call

- severity: Critical
- files: `shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-grpc/src/main/java/org/apache/shenyu/plugin/grpc/GrpcPlugin.java:107`; `.../grpc/client/ShenyuGrpcClient.java:95`
- description: `doExecute` calls `client.call(...)` which internally does `this.invoke(callParams).get()` — a blocking `ListenableFuture.get()`. `Mono.fromFuture(result.thenApply(...))` at line 110 wraps an already-completed future (line 108 returns `CompletableFuture.completedFuture(...)`), providing no async scheduling. The entire gRPC RPC blocks the reactor netty event-loop thread for up to `extInfo.timeout` (default 5000ms).
- impact: Starves all other requests sharing that event-loop thread; under load causes cascading timeouts and thread starvation.
- suggested_fix: `Mono.fromCallable(() -> client.call(...)).subscribeOn(Schedulers.boundedElastic())` or make `ShenyuGrpcClient.call` return the uncompleted future without `.get()`.
- confidence: High
- related_existing: none — PERF-45 (ClickHouse `.get()`) is a different module; this is the gRPC plugin's own blocking call on the event loop.

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## High

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/00-consolidated-critical-high.md`](docs/scan2-2026-08-02/00-consolidated-critical-high.md)._

Start with GrpcPlugin.java:107-110 and ShenyuGrpcClient.java:95, tracing doExecute through client.call(...) and the blocking invoke(...).get() call. Compare the suggested scheduling or future-handling approaches, then verify that the gRPC RPC no longer blocks the Reactor Netty event-loop thread for the timeout period.

Written by the indexing model from the issue text.