* fix: dispatch execution_end hook on failed crew and flow executions
The `execution_end` interception point only fired after a successful
kickoff, so consumers never learned about failed runs. Crew kickoff
paths (`kickoff`/`akickoff`) and the flow runtime (`kickoff_async`,
`resume_async`) now dispatch it on the failure path too, with new
additive `status` ("completed"/"failed") and `error` fields on
`ExecutionEndContext`. Pairing flags guarantee exactly-once dispatch,
keep the start/end pairing invariant, and the original exception
propagates unchanged.
* fix: track execution_end pairing per invocation for reentrant flows
Reentrant kickoffs on the same Flow instance are supported (usage
aggregation already accommodates them), but the instance-level pairing
booleans let an inner kickoff's completion mark the outer execution as
ended, skipping the outer failure's `execution_end`. The pairing state
now lives in each `kickoff_async` invocation's locals, and the resume
path passes a per-invocation holder into `_resume_async_body`. Crew
keeps its instance flags since crew kickoffs are not reentrant on the
same instance (`kickoff_for_each` copies the crew).
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