Strip underscore-prefixed keys from logging redaction

[anth-volk]

Fixes #640

What

redact_params_for_logging now drops all underscore-prefixed keys, not just the hardcoded _telemetry/_metadata. Correlation ids are still surfaced explicitly.

Why

Merging #637 turned segmented national on, and the deploy failed at beta integration: every segmented-national child (run_simulation_segment) crashed before running any simulation, in its observability wrapper —

File "/root/app.py", line 324, in run_simulation_segment
ValueError: Attribute keys cannot start with an underscore.

Child payloads carry _emit_microdata: True (required — it drives the child's microdata emission for the reduce). run_simulation_segment splats redact_params_for_logging(params) into operation(..., **redacted_params); _emit_microdata survived redaction and reached the observability backend as an attribute key, which the backend rejects. The parent (run_simulation) never carries _emit_microdata, so only the children crashed — the fan-out mechanics and fail-fast otherwise worked. Prod was gated (skipped) and is unaffected; main's deploy pipeline is red until this lands.

#637's staging validation (C7) missed it because the throwaway benchmark's run_simulation_segment called run_simulation_impl directly with no observability wrapper — the exact wrapper that fails.

Fix scope

One rule in logging_redaction.py (not key.startswith("_")) covers all three splat sites (run_simulation, run_simulation_segment, run_budget_window_batch), and future-proofs any new internal _-key. redacted_params feeds only observability attributes — nothing functional depends on it; the real params reach run_simulation_impl untouched.

Tests

• test_app_redaction.py: new regression asserting no _-prefixed key survives redaction while routing context + run_id are preserved (4 pass).
• Full confirmation of the child path is the deploy pipeline's national integ test, fail-closed before prod. After this, the child runs the exact code path C7 validated end-to-end (microdata emit + reduce, bit-exact).

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