feat(eventbus): support per-subscriber async dispatch with graceful drain

[WyRainBow]

Background

EventBus previously dispatched events synchronously. Slow subscribers could block Send() and impact the whole event pipeline.

What Changed

• Added optional async subscriber support via AsyncSubscriber.
• Refactored EventBus subscriber directory to maintain per-subscriber runtime state.
• Added per-subscriber async queue and drainer lifecycle control.
• Added lifecycle guard and graceful integration changes:
  • compile-time graceful component contract check
  • safer config validation path in AdminConfig
  • default eventbus buffer size aligned to 1024

Core Files

• pkg/core/events/async.go
• pkg/core/events/component.go
• pkg/config/app/admin.go
• pkg/config/eventbus/config.go

Behavior Summary

• Async subscribers: non-blocking enqueue (select + default), drop with warn when channel is full.
• Sync subscribers: keep existing synchronous ProcessEvent behavior.
• Unsubscribe and shutdown paths keep graceful drain semantics.

Notes

• This PR intentionally includes only core feature/code path updates.
• Test/documentation local changes are not included in this PR commit scope.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

Adds opt-in asynchronous dispatching to the core EventBus to prevent slow subscribers from blocking the event pipeline, and wires graceful draining into runtime shutdown.

Changes:

• Introduces AsyncSubscriber and per-subscriber runtime state with per-subscriber buffered queues + drainer goroutines.
• Integrates EventBus graceful drain into runtime stop handling via runtime.GracefulComponent.
• Adds eventBus configuration to AdminConfig and increases the default EventBus buffer size to 1024.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/core/runtime/runtime.go	On stop, waits for GracefulComponent implementations to finish (enables graceful drains).
pkg/core/events/component.go	Refactors subscriber directory to track per-subscriber state; supports async enqueue + drainer lifecycle; adds graceful drain (WaitForDone).
pkg/core/events/async.go	Adds AsyncSubscriber interface and subscriberState for async runtime bookkeeping.
pkg/core/engine/subscriber/runtime_instance.go	Marks runtime instance subscriber as async-capable (AsyncEnabled).
pkg/core/discovery/subscriber/zk_metadata.go	Marks ZK metadata subscriber as async-capable (AsyncEnabled).
pkg/core/discovery/subscriber/zk_config.go	Marks ZK config subscriber as async-capable (AsyncEnabled).
pkg/core/discovery/subscriber/service_provider_metadata.go	Marks service provider metadata subscriber as async-capable (AsyncEnabled).
pkg/core/discovery/subscriber/service_consumer_metadata.go	Marks service consumer metadata subscriber as async-capable (AsyncEnabled).
pkg/core/discovery/subscriber/rpc_instance.go	Marks RPC instance subscriber as async-capable (AsyncEnabled).
pkg/core/discovery/subscriber/nacos_service.go	Marks Nacos service subscriber as async-capable (AsyncEnabled).
pkg/core/discovery/subscriber/instance.go	Marks instance subscriber as async-capable (AsyncEnabled).
pkg/config/eventbus/config.go	Raises default EventBus buffer size to 1024.
pkg/config/app/admin.go	Adds EventBus config field, defaults, and validation wiring.

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