fix(plan): ensure changeHook receives finished plan instead of null in finishPlan()

[birdie7761]

Description

PlanNotebook.finishPlan() sets currentPlan = null before calling triggerPlanChangeHooks(), causing all registered change hooks to receive null instead of the finished plan. This prevents downstream event emission (e.g.
TASK_EXECUTION_SUCCEEDED, TASK_EXECUTION_CANCELLED) and makes getCurrentPlan() return null during hook execution.

All other mutation methods (createPlan, updatePlanInfo, finishSubtask, etc.) trigger hooks while currentPlan still references a valid object. finishPlan is the only method that nulls before hooking.

Root cause — PlanNotebook.java:

// Before fix: null first, then notify
currentPlan = null;
return triggerPlanChangeHooks()

triggerPlanChangeHooks() reads this.currentPlan directly, so hooks receive null.

Fix: trigger hooks first, then null currentPlan:

return storage.addPlan(currentPlan)
        .then(triggerPlanChangeHooks())
        .then(Mono.fromRunnable(() -> currentPlan = null))
        .thenReturn(message);

Checklist

- Code has been formatted with mvn spotless:apply
- All tests are passing (mvn test)
- Javadoc comments are complete and follow project conventions
- Related documentation has been updated (e.g. links, examples, etc.)
- Code is ready for review

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All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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