[BUG] NPE when metaData rpcExt is null or blank

- severity: High
- files: `shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-grpc/src/main/java/org/apache/shenyu/plugin/grpc/GrpcPlugin.java:102-103`
- description: `GsonUtils.getGson().fromJson(metaData.getRpcExt(), GrpcExtInfo.class)` returns null `extInfo` when `rpcExt` is null/blank. Line 103 dereferences `extInfo.timeout` → NPE. `extInfo.methodType` (line 107) also null.
- impact: Any gRPC metadata registered without `rpcExt` causes an uncaught NPE.
- suggested_fix: Null-check `extInfo` after deserialization; supply a default `GrpcExtInfo`.
- confidence: High
- related_existing: none — #6490/#6492 are ingress parsing, not runtime plugin.

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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 in shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-rpc/shenyu-plugin-grpc/src/main/java/org/apache/shenyu/plugin/grpc/GrpcPlugin.java at lines 102-103, and inspect GrpcExtInfo to understand the default values. Verify that metadata with a null or blank rpcExt no longer causes an NPE when timeout or methodType is accessed, then run the relevant gRPC plugin tests if available.

Written by the indexing model from the issue text.