Soft-skip AI steps that produce no actionable output

[chubes4]

Summary

• When the AI step's conversation loop completes without an error but emits no actionable output (no handler tool, no other tool result, no AI text content), set a job_status engine override of agent_skipped - ai_step_no_actionable_output instead of falling through to a generic empty-packet failure.
• Engine routing then completes that child via the existing status-override branch, so parent batches no longer roll those children up as failed - All N child jobs failed.
• The new behavior is gated by a datamachine_ai_step_soft_skip_on_empty_output filter (default true) so any pipeline that genuinely needs a hard-fail on empty AI output can opt out without re-pinning the old behavior in core.

Why

For autonomous brain pipelines, the dominant outcome of any source search is: most fetched items don't deserve a wiki page. The pipeline's own system prompt says "skip irrelevant, too-thin, or transient chatter instead of writing filler." Today, every one of those skips renders as failed - empty_data_packet_returned, trips parent batches into failed, and discourages enabling additional source lanes.

This change makes the model's intended skip path the operational skip path.

Fixes #1789.

Testing

• php tests/ai-step-soft-skip-empty-output-smoke.php (new) — asserts the override completes the job as agent_skipped, persists the soft-skip reason, and does NOT emit datamachine_fail_job. Also asserts the no-override path still fails with empty_data_packet_returned.
• php tests/ai-error-status-propagation-smoke.php — existing AI failure status propagation still passes.
• php tests/wp-ai-client-tool-schema-smoke.php
• php tests/wp-ai-client-request-timeout-smoke.php
• php -l inc/Core/Steps/AI/AIStep.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Locating the empty-packet fail path, designing the override-based soft-skip, drafting the AIStep change and the new smoke test, and running focused verification. Chris remains responsible for review and merge.