[MNG-8547] Introduce Event/Listener hierarchy with ExecutionEvent and ExecutionListener

[gnodet]

Description

Introduce a base Event/Listener hierarchy to support multiple event families (execution, repository, etc.) through a single registration point on Session.

This PR refactors the existing Event/Listener/EventType types and introduces new typed specializations for build lifecycle events. The design enables PR #13011 to integrate its RepositoryEvent/RepositoryListener into the same hierarchy by simply extending Event and Listener.

API changes

Type	Change
Event	Stripped to base marker: @Immutable, single session() noun-style accessor
ExecutionEvent	New — typed execution event (extends Event) with type(), project(), mojoExecution(), exception()
ExecutionEventType	New — renamed from EventType for clarity, same values in same order
EventType	@Deprecated(forRemoval) — alias with bridge methods to ExecutionEventType
ExecutionListener	New — typed callbacks (extends Listener) with per-event-type default methods
Listener	Refactored to base marker, @FunctionalInterface removed, onEvent() deprecated as default

Impl changes

• DefaultEvent: implements ExecutionEvent with noun-style accessors
• EventSpyImpl: dispatches to both legacy Listener.onEvent() and typed ExecutionListener callbacks

Design

Event (@Immutable, base marker)
├── ExecutionEvent (project, mojo, session lifecycle)
└── RepositoryEvent (artifact/metadata — PR #13011)

Listener (@Consumer, base marker)
├── ExecutionListener (typed default callbacks)
└── RepositoryListener (typed default callbacks — PR #13011)

Key properties:

• Single registration: Session.registerListener(Listener) handles all listener types — no method overloading
• Extensible: future event categories just add XxxEvent extends Event + XxxListener extends Listener
• Backward compatible: old Listener.onEvent() stays as @Deprecated default
• Noun-style accessors on new types, consistent with Switch core API value types to noun-style accessors #13036
• Session.java, AbstractSession.java, SessionStub.java are unchanged — the existing List<Listener> works for both old and new listener types

Relationship with PR #13011

This PR provides the hierarchy foundation. PR #13011 should merge this, then:

1. Make RepositoryEvent extends Event (add session() accessor)
2. Make RepositoryListener extends Listener
3. Remove the separate registerListener(RepositoryListener)/getRepositoryListeners() methods from Session
4. Use noun-style accessors on RepositoryEvent and RepositoryMetadata

---

• This pull request addresses one issue without unrelated changes.

• The description explains what the pull request does, how, and why.

• The commit has a meaningful subject and body.

• mvn verify passes.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

[gnodet]

Solid API design — the hierarchy cleanly separates execution events from the base marker, and the backward compatibility story is well thought out (deprecated bridge methods, dual dispatch). Two minor observations below.

This review was generated by an AI agent, Hermès on behalf of @gnodet.

[gnodet]

💡 Suggestion (low): The default branch is unreachable — all 17 ExecutionEventType values are explicitly handled. Removing it turns this into an exhaustive switch, so the compiler will flag any future enum additions as a compile error rather than silently dropping them.

Suggested change

	case MOJO_SUCCEEDED -> listener.mojoSucceeded(event);
	case FORKED_PROJECT_FAILED -> listener.forkedProjectFailed(event);

[gnodet]

💡 Suggestion (low): @since 4.0.0 is misleading — this type is new in 4.1.0. EventType existed in 4.0.0, but ExecutionEventType didn't. The second @since tag clarifies the rename, but the first tag should match the actual introduction version.

Suggested change

	*/
	* @since 4.1.0 (renamed from {@link EventType})