[reginaldalfret]

Summary

Fixes #1131.

In BaseAgent.handleBeforeAgentCallback and BaseAgent.handleAfterAgentCallback, execution previously only checked his.canonicalBeforeAgentCallbacks / his.canonicalAfterAgentCallbacks. If an agent had no instance callbacks configured, it would return undefined immediately without invoking registered plugins. Furthermore, even when instance callbacks were present, invocationContext.pluginManager.runBeforeAgentCallback and
unAfterAgentCallback were never invoked.

This PR connects BaseAgent callback handling to PluginManager:

1. In handleBeforeAgentCallback, invokes invocationContext.pluginManager.runBeforeAgentCallback({agent: this, callbackContext}). If a plugin returns Content, execution short-circuits (invocationContext.endInvocation = true) and emits the returned content. If no override is provided, canonical instance callbacks run.
2. In handleAfterAgentCallback, invokes invocationContext.pluginManager.runAfterAgentCallback({agent: this, callbackContext}). If a plugin returns Content, it yields the replacement content event. If no override is provided, canonical instance callbacks run.
3. If either plugin callback mutates state, state delta events are properly emitted.

Verification

pm run build completed successfully across all workspaces.

pm run ts:check completed with 0 errors.

pm run format:check confirmed Prettier code style passes.

pm run lint completed with 0 errors.

• Unit tests in core/test/agents/base_agent_test.ts (39/39 passing) and all plugin tests in core/test/plugins/ (267/267 passing).

[hirematha]

Hi @reginaldalfret, Thanks for your pull request!
Removing the early return completely causes new Context({invocationContext}) to run even when no plugins or agent callbacks are configured, which breaks 7 tests in core/test/agents/routed_agent_test.ts (TypeError: Cannot read properties of undefined (reading 'state')).
Could we keep the early return when neither plugins (!invocationContext.pluginManager?.hasPlugins) nor canonical agent callbacks are present?