Remove support for string-encoded dataclips in step:complete

[josephjclark]

DO NOT MERGE

Description

In #5098 we added functionality to support dataclips being sent as JSON (where as previously they were sent as a JSON encoded string)

This PR removes the "maybe" and forces step:complete to treat the dataclip as an object.

We cannot merge this until we've had a major bump of the worker and found some resolution to #5103

Validation steps

1.Run a workflow

1. Ensure that the output of each step is correct (not an object with { value } wrapper)
2. Ensure that scalar (number, string, boolean) return value are all handled with { value } wrapper

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[github-actions]

Security Review ✅

• S0 (project scoping): step:complete still writes the dataclip with project_id derived authoritatively from the worker socket (lib/lightning_web/channels/run_channel.ex:207-210); the wire-format change to output_dataclip (lib/lightning/runs/handlers.ex:422,554-557) does not alter scoping.
• S1 (authorization): N/A — no new controllers, LiveView events, channel handlers, or policy modules; only the shape of an existing worker payload field changed.
• S2 (audit trail): N/A — the only affected writes are :step_result dataclip inserts (runtime execution data), not project/instance configuration, so no audit entry is expected.