fix: make action invocation retries idempotent (#355)

* fix: make action invocation retries idempotent

* fix: persist idempotency claim before action dispatch

* test: model failed idempotency result write

* fix: preserve action handler on idempotent replay

* fix: snapshot action handler for replay

* fix: snapshot action node before provider send

* fix: await native spawn placement on replay

* fix: wait for durable spawn replay outcome

* fix: wait for keyed action replay outcome

* fix: classify idempotent replays from durable claims

* fix: gate agent replays on dispatch start

* fix: classify insert-race replays from claims

* fix(engine): close provider dispatch takeover race

* fix(engine): require owner-authorized agent dispatch

* fix(engine): release failed spawn placement claims

---------

Co-authored-by: kjgbot <kjgbot@agentrelay.dev>