fix(workflow): make background execution failures graceful

[daryllimyt]

Summary

• Update create_workflow_execution_nowait and create_draft_workflow_execution_nowait to schedule only workflow start acknowledgement in the background.
• Add guarded background start wrappers that catch and log start-time failures.
• Keep synchronous wait paths unchanged.
• Add unit tests for nowait start-only behavior and exception swallowing.

Problem

Background fire-and-forget dispatches were waiting on full workflow completion. Expected workflow/action failures could then surface as noisy:

Task exception was never retrieved

Validation

• uv run ruff check tracecat/workflow/executions/service.py tests/unit/test_workflow_executions.py
• uv run pyright tracecat/workflow/executions/service.py tests/unit/test_workflow_executions.py
• TRACECAT__SERVICE_KEY=dummy uv run pytest tests/unit/test_workflow_executions.py -q

[cubic-dev-ai]

No issues found across 2 files