Fix experimental queue metrics hanging with missing body size

[tewaro]

No description provided.

[ask-bonk]

The fix is consistent with existing patterns. The analysis is clear:

1. The bug: kj::HttpClient::request() for GET without passing an expectedBodySize leaves the body stream open (since kj::none means "unknown body size"). The mock service in the test reads request.arrayBuffer(), which deadlocks because the body pipe never closes.
2. The fix: Pass static_cast<uint64_t>(0) to signal that the GET request has zero body bytes, so the body pipe is immediately closed.
3. The test: The test now reads the request body before responding, which matches production behavior and would have caught this hang.

This is a clean, correct, minimal fix. The pattern matches existing usage in r2-rpc.c++:121. The test properly validates the regression.

LGTM

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