test: fix flaky tests caused by worker thread pollution (#88)

## 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:
```ruby
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

<!-- CURSOR_SUMMARY -->
---

> [!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.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
27e0006. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: kinyoklion

spec/client_spec.rb

@@ -32,6 +32,8 @@ def with_client(client)
       yield client
     ensure
       client.close
+      # Wait for SSE worker thread to terminate before next test reuses the port
+      Thread.list.select { |t| t.name == 'LD/SSEClient' }.each { |t| t.join(1) }
     end
   end
 
@@ -1124,7 +1126,7 @@ def test_object.to_s
           requests << request_data
           attempt += 1
           if attempt == 1
-            send_stream_content(res, "", keep_open: false)  # Close to trigger reconnect
+            send_stream_content(res, ": keepalive\n\n", keep_open: false)  # Close to trigger reconnect
           else
             send_stream_content(res, "", keep_open: true)
           end
@@ -1334,9 +1336,7 @@ def test_object.to_s
           requests << request_data
           attempt += 1
           if attempt <= 2
-            res.status = 500
-            res.body = "error"
-            res.keep_alive = false
+            send_stream_content(res, ": ping\n\n", keep_open: false)  # Close to trigger reconnect
           else
             send_stream_content(res, "", keep_open: true)
           end

spec/headers_spec.rb

@@ -18,6 +18,8 @@ def with_client(client)
       yield client
     ensure
       client.close
+      # Wait for SSE worker thread to terminate before next test reuses the port
+      Thread.list.select { |t| t.name == 'LD/SSEClient' }.each { |t| t.join(1) }
     end
   end