Remove the None presets and checks around Stage 12 span and operation blocks #964

Before policyengine-observability 3.0.4, the scope managers that runtime.span(...) and runtime.operation(...) return annotated __exit__ and __aexit__ as returning bool, so Pyright read every span or operation block as possibly suppressing its exception. #712 worked around that in two executor modules by setting each value a block assigns to None before the block and raising a RuntimeError after it when the value was still None:

• projects/policyengine-simulation-executor/src/modal/v2_app.py: run_single_simulation_segment_us, run_single_simulation_us, run_single_simulation_uk and coordinate_report each set result = None before their runtime.operation(...) block and raise "... returned no result" after it.
• projects/policyengine-simulation-executor/src/policyengine_simulation_executor/stage12_runtime/fanout.py: Stage12SegmentRunner.run sets call = None before each dispatch span and passes the call through _require_call, which raises "Stage 12 segment dispatch returned no call", and sets payloads and merged to None before the wait and merge spans and raises "... returned no payloads" and "... returned no calculation" after them.

#959 moved every package to 3.0.4, whose scopes are annotated as returning Literal[False] and return False on every path, so each block either assigns its value or its exception reaches the caller. None of the assigned values can be None either: calculate_segment(...).model_dump(...) is a dict, run_single_simulation and coordinate_report are typed dict[str, Any] and return a dict on every path, modal 1.4.2's Function.spawn always returns a FunctionCall, _collect returns a list or raises, and merge_segment_results returns a SimulationCalculation. The presets and checks are therefore unreachable code.

A search of the executor, gateway and entrypoint src directories found no other value set to None before a span or operation block and checked after it. setup_gcp_credentials sets creds_file = None before an ExitStack block that may finish without assigning it, so that one is a real check and stays.

This issue covers:

• tests that run each of those blocks through the scopes of the runtime the services build, passing before and after the removal;
• removing the presets, the checks and _require_call, with Pyright still reporting no errors.