NIFI-16104 Fix flaky ConnectorAssetsIT by polling for asset removal

[pvillard31]

Summary

ConnectorAssetsIT.testCreateConnectorAndUploadAsset intermittently fails with:

ConnectorAssetsIT.testCreateConnectorAndUploadAsset:199 expected: <false> but was: <true>

The test asserts that an unreferenced asset is removed immediately after waitForConnectorState(STOPPED). Asset cleanup is asynchronous and may not have completed by the time the connector reaches STOPPED state, making the single-shot assertion inherently racy.

Changes

• NiFiClientUtil — add waitForAssetRemoved(connectorId, assetId) utility that polls every 100 ms (matching the waitForConnectorState pattern) until the specified asset is absent from the connector's asset list.
• ConnectorAssetsIT — replace the immediate one-shot getAssets / assertFalse check with a call to waitForAssetRemoved, tolerating the async cleanup delay.

Tracking

Issue Tracking

• Apache NiFi Jira issue linked

Pull Request Tracking

• Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-16084
• Pull Request commit message starts with Apache NiFi Jira issue number, as such NIFI-16084

Testing

• Existing testCreateConnectorAndUploadAsset now reliably passes because it waits for the async asset cleanup instead of checking exactly once.