When a workspace delegated a task to itself, it would acquire
_run_lock twice on the same goroutine mutex, blocking permanently.
Add an early-return guard in `DelegationHandler.Delegate` that
returns HTTP 400 {"error": "self-delegation not permitted"} as soon
as sourceID == body.TargetID, before any DB or A2A work is done.
Adds TestDelegate_SelfDelegation_Rejected to delegation_test.go.
Closes#548
Co-authored-by: Molecule AI Backend Engineer <backend-engineer@agents.moleculesai.app>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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