test(qwp): fix port collision flake in walk tracker test

[bluestreak01]

Test-only change. It fixes an intermittent failure of QwpQueryClientWalkTrackerTest.testWalk_TransportFailureContinuesWalk.

Failure

The test failed on questdb/questdb#7680 CI, job "SelfHosted Other tests (B) on linux-x64-zfs" (build 274943):

java.lang.IllegalArgumentException: duplicate addr entry: localhost:46809
	at io.questdb.client.impl.ConfigView.parseEntry(ConfigView.java:327)
	at io.questdb.client.impl.ConfigView.getHostPorts(ConfigView.java:169)
	at io.questdb.client.cutlass.qwp.client.QwpQueryClient.validateConfig(QwpQueryClient.java:504)
	at io.questdb.client.cutlass.qwp.client.QwpQueryClient.fromConfig(QwpQueryClient.java:387)
	at io.questdb.client.test.cutlass.qwp.client.QwpQueryClientWalkTrackerTest.testWalk_TransportFailureContinuesWalk(QwpQueryClientWalkTrackerTest.java:451)

Root cause

The test builds addr=localhost:<portDead>,localhost:<portOk>:

• TestPorts.findUnusedPort() supplies portDead: it binds port 0, closes the probe socket, and returns the number. Nothing holds the port afterwards.
• new TestWebSocketServer(NOOP_HANDLER) then binds its own port 0 for portOk, and the kernel may hand it the port the probe just released.

In the failing run both values came out as 46809. ConfigView.parseEntry() correctly rejects the duplicate (host, port) pair, so fromConfig() threw before the client attempted any connection. The walk-tracker logic the test targets never ran, and the product code behaved as specified.

The TestPorts.findUnusedPorts(int) javadoc already describes the same race for two back-to-back findUnusedPort() calls. This test pairs one probe with a server's own bind(0), which that helper does not cover.

Change

The test now creates the server first and probes for portDead while the server's listener holds portOk. TestWebSocketServer binds its listener in the constructor and keeps it until close(), and the kernel never assigns a bind(0) port that a live socket on the same address holds, so the two ports cannot collide.

Tradeoff: the test keeps the suite's existing bind-close-reuse exposure, where another process could bind portDead between the probe and the client's connect attempt. Every other pre-selected-port test in the suite accepts the same exposure. The change touches no production code and no shared test helper.

Test plan

• mvn -pl core test -Dtest=QwpQueryClientWalkTrackerTest: 12/12 green on JDK 25, both on 0b9b5766 (the commit questdb master pins) and on this branch (main plus the fix).
• A green run cannot prove the absence of a rare random collision; the fix relies on kernel bind semantics rather than on timing.
• Not built on JDK 8; the change only reorders existing calls and adds a comment.