fix(campaign): mid-run event injection actually reaches agents (#29 A1/A2)

[PRATHAMESH75]

Summary

Part of #29 — fixes the two P0 correctness bugs:

• A1 — event injection never lands. run_parallel_simulation.py only started servicing IPC interview commands (env_status.json → alive) after the entire round loop finished. Since the competitive/crisis event is scheduled mid-run (round 7/14 by default), every injection attempt failed with Simulation environment is not running or has closed, cannot perform Interview, and scenarios A/B/C ran on identical inputs. Fixed by starting a command dispatcher and registering each platform's env with ParallelIPCHandler as soon as it's constructed (before the round loop begins), instead of waiting until both platforms finish. A per-platform asyncio.Lock is shared between the round loop and the interview dispatcher so an interview's env.step() can never run concurrently with the round loop's own env.step() on the same OASIS env.
• A2 — failure reported as success. POST /campaign/<id>/inject always returned success: true regardless of whether the interview actually reached any agents, and the auto/staggered injection path in campaign_manager.py marked scenario_b_injected/scenario_c_injected = True even when the interview call reported failure (only exceptions were checked, not the result's own success flag). Both paths now check result.get("success") before marking anything as injected, and the API route returns success: false (HTTP 502) when the underlying injection failed, instead of silently reporting success on a no-op.

Test plan

• uv run pytest — 127 passed (backend)
• New test_parallel_simulation_ipc.py: verifies register_twitter/register_reddit mark the env alive immediately, and that interview-triggered env.step() never overlaps the round loop's own env.step() on the same env (regression guard via a lock-asserting fake env)
• New test_campaign_injection_success.py: inject_scenario_event and _execute_pending_injections only flip scenario_*_injected on actual success
• test_sentiment_routes.py: /inject route propagates failure as success: false / HTTP 502 instead of always success: true

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