chore: Fix flaky unit test that rely on lazy loaded req

[jsonbailey]

Several properties are lazy loaded in the req object. If we want to access their properties later it introduces a race condition. We should copy all information during the request that we want to use later.

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Note

Stabilize flaky SSE client specs

• Replace queued req objects with snapshot request_data hashes (e.g., headers, method, body) captured before closing the stream to avoid lazy-loaded property races
• Update expectations to reference request_data (e.g., received_req[:headers], [:method], [:body]), including reconnect Last-Event-Id and payload-as-JSON assertions
• Minor cleanups: add explanatory comments and variable renames (e.g., _req1) to clarify unused values

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