test(rpcinfo): cover frozen RPCInfo after recycle

[Haihan-Jiang]

What

Adds regression coverage for the documented async-safe FreezeRPCInfo path related to #1958. The test freezes an RPCInfo, returns the original pooled value with PutRPCInfo, and verifies that the frozen copy still retains endpoint, invocation, and timeout configuration values while omitting stats.

Why

Issue #1958 discusses the risk of keeping GetRPCInfo(ctx) references past the request lifecycle when rpcinfo pooling is enabled. This test locks down the expected behavior for the safer frozen snapshot path.

Tests

• go test ./pkg/rpcinfo
• go test -race ./pkg/rpcinfo

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