test: fix EventCollector double-free race in sse client tests

[beekld]

Summary

ASAN occasionally caught a double-free in ClientTest.OnConnectHookSeesPreviousMutations (and potentially other sse-client tests) on CI. We couldn't reproduce locally — even on Linux — and it appears to fire only rarely on CI.

The fixture declared IoContextRunner before EventCollector, so on scope exit the collector was destroyed first while the io_context worker could still be running a receiver callback that pushed into the collector's vector. Swap declaration order so the collector outlives the runner — ~IoContextRunner joins the worker, so once it returns no callback can race the collector's destructor.

Test plan

• Full sse-client suite (101 tests, 1 pre-existing skip) green under ASAN locally

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Note

Low Risk
Test-only declaration order and documentation; no runtime library or API changes.

Overview
Fixes a flaky ASAN double-free in SSE client integration tests by correcting stack object destruction order in test fixtures.

Tests previously declared IoContextRunner before EventCollector, so on scope exit the collector was destroyed while the background io_context thread could still run receiver lambdas that write into it. The change declares EventCollector first, then IoContextRunner, so ~IoContextRunner joins the worker before the collector is torn down. A short comment on IoContextRunner documents this requirement for future tests.

No production SSE client behavior changes—test-only lifetime fix applied across the affected ClientTest cases in client_test.cpp.

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[kinyoklion]

The OnConnectHookInvokedBeforeRequest and OnConnectHookInvokedOnEachReconnect tests violate the invariant that any state captured must be declared before the runner, but in a way that is harmless.