feat: allow building ConductorWorkflow without an executor

[Apeksha9389]

Fixes #432

ConductorWorkflow required a live WorkflowExecutor, so building or serializing a definition (e.g. in unit tests) needed a server. executor is now optional (the positional order is unchanged and name is still required). register(), start_workflow(), start_workflow_with_input() and execute() raise a RuntimeError pointing to executor= when none was given.

Testing: added tests/unit/workflow/test_conductor_workflow_offline.py (6 tests: offline to_workflow_def(), name still required, the three executor-backed methods raise, positional executor still used); 4 fail without the change. Unit suite: 2566 passed; the 17 failures in tests/unit/ai are the same on main.

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