* fix(flow): add execution_id separate from state.id (COR-48)
When a consumer passes `id` in `kickoff(inputs=...)`, that value
overwrites the flow's state.id — which was also being used as the
execution tracking identity for telemetry, tracing, and external
correlation. Two kickoffs sharing the same consumer id ended up
with the same tracking id, breaking any downstream system that
joins on it.
Introduces `Flow.execution_id`: a stable per-run identifier stored
as a `PrivateAttr` on the `Flow` model, exposed via property +
setter. It defaults to a fresh `uuid4` per instance, is never
touched by `inputs["id"]`, and can be assigned by outer systems
that already have an execution identity (e.g. a task id).
Switches the `current_flow_id` / `current_flow_request_id`
ContextVars to seed from `execution_id` so OTel spans emitted by
`FlowTrackable` children correlate on the stable tracking key.
`state.id` keeps its existing override semantics for
persistence/restore — consumers resuming a persisted flow via
`inputs["id"]` work exactly as before.
Adds tests covering default uniqueness per instance, immunity to
consumer `inputs["id"]`, context-var propagation, absence from
serialized state, and parity for dict-state flows.
Co-authored-by: Greyson LaLonde <greyson.r.lalonde@gmail.com>
