[Bug]: HttpSelfRegistration.close() skips deregistration when a heartbeat re-register cycle is in flight (flaky HttpSelfRegistrationTest on CI)

## Description

While CI-testing PR #3131 on `windows-latest`, `HttpSelfRegistrationTest.
registrationUsesTenantAndDurableIdentityForHeartbeatAndDelete` failed
intermittently with the `deleted` latch timing out at
`HttpSelfRegistrationTest.java:106`:

```
org.opentest4j.AssertionFailedError: expected: but was:
at io.agentscope.extensions.aistio.transport.HttpSelfRegistrationTest
.registrationUsesTenantAndDurableIdentityForHeartbeatAndDelete(...:106)
```

The runner stderr showed `aistio: registered runtime-instance-key ...` **twice**,
indicating a heartbeat-failure re-register cycle ran concurrently with
`close()`.

## Root cause

`HttpSelfRegistration.close()` gates the control-plane DELETE on
`registered.get()`:

```java
if (!registered.get()) {
return; // skips the DELETE even if registeredInstanceId is still set
}
```

`heartbeatSafe()` clears `registered` **before** re-registering after a
heartbeat failure. If `close()` runs inside that window (likely on loaded
runners where a heartbeat request can fail), the DELETE is silently skipped:
the instance is leaked on the control plane, and the test's `deleted` latch
times out. Fast local machines rarely hit the window, which is why the test
is green locally and only flakes on CI.

## Impact

- Real resource leak: an instance that was registered stays registered on the
control plane after `close()` (ghost instance).
- Flaky CI: `HttpSelfRegistrationTest` fails under load.

## Suggested fix

Make `close()` best-effort unregister whenever an identity exists
(`registeredInstanceId != null`), keeping never-registered calls as a no-op
and repeat closes idempotent; widen the test latches from 2s to 10s so slow
runners don't turn slow (not stuck) callbacks into failures.

Fix will be attached to PR #3131 (discovered by its CI run).

Start with HttpSelfRegistration.close() and heartbeatSafe(), then inspect HttpSelfRegistrationTest.registrationUsesTenantAndDurableIdentityForHeartbeatAndDelete(), which reports the failure at line 106. Run that test under the relevant CI conditions and trace the heartbeat re-registration window. Done means never-registered closes remain no-ops, repeat closes are idempotent, an existing registeredInstanceId is deleted, and the test tolerates slow callbacks with the requested latch timeout.

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