test: fix flaky tests caused by worker thread pollution

[kinyoklion]

Summary

Fixes flaky test failures across client_spec.rb and headers_spec.rb caused by SSE worker thread pollution between tests.

Root cause: SSE::Client spawns a fire-and-forget worker thread (Thread.new { run_stream }) that is not joined on close. After client.close sets @stopped, the worker thread may still complete one more reconnection attempt before checking the flag. Since all tests reuse port 50000, this phantom request hits the next test's WEBrick server — arriving without query params or with stale headers, causing nil assertions.

Fix: In both with_client helpers, join the named 'LD/SSEClient' thread after close:

Thread.list.select { |t| t.name == 'LD/SSEClient' }.each { |t| t.join(1) }

Also uses EOF-based reconnection (stream close) instead of HTTP 500 responses for the multi-reconnection test, matching the pattern of all stable tests.

Verified: 5/5 CI passes, 20/20 local runs on Ruby 3.2, 20/20 on Ruby 3.3.

Link to Devin session: https://app.devin.ai/sessions/e40ffc3296dd45648f86ac10c42b6da5
Requested by: @kinyoklion

---

Note

Low Risk
Spec-only changes; no production library behavior is modified.

Overview
Stabilizes end-to-end specs in client_spec.rb and headers_spec.rb by ensuring each example fully tears down the SSE::Client worker before the shared stub server port is reused.

Both with_client helpers now join(0.01s)** any thread named **LD/SSEClient** after **client.close`, so a lingering reconnect cannot hit the next example’s WEBrick handler with wrong query strings or headers.

Two dynamic query_params reconnection examples no longer force reconnect via HTTP 500; they close the event stream (minimal SSE comment payloads) instead, aligning with other stable reconnect tests.

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