[BUG] AbstractWasmDiscoveryHandler does not forward upstream deletion events #7562

Is there an existing issue for this?

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Current Behavior

AbstractWasmDiscoveryHandler implements only handlerDiscoveryUpstreamData.

CommonDiscoveryUpstreamDataSubscriber dispatches deletion through removeDiscoveryUpstreamData(DiscoveryUpstreamKey). Because the WASM handler does not override it, it inherits the interface's empty implementation.

A guest that maintains discovery state receives updates but receives no deletion notification through this base class.

Expected Behavior

Forward deletion to a documented guest callback, including the DiscoveryUpstreamKey data needed to remove the relevant state. Define behavior for guests that do not implement this callback.

Steps To Reproduce

Source-derived scenario:

1. Implement a WASM discovery handler using the existing base class, without a custom Java deletion override.
2. Deliver an upstream update and store it in guest state.
3. Deliver a deletion through the discovery subscriber.
4. No guest deletion callback is invoked; guest-maintained state can retain the removed upstream.

Environment

ShenYu: 2.7.2-SNAPSHOT, commit 2dff2730ff399635b2bc657e21a903f391fb9df8; Java 17; Chicory 1.7.3.

Anything else?

Current Go/Rust discovery tests cover updates only. Add deletion coverage through both the handler and subscriber, including a missing optional export case.