Add simulation API observability coverage

[anth-volk]

Fixes #593

Summary

• Add policyengine-observability initialization and structured logging for simulation Modal gateway and worker paths.
• Keep legacy Logfire enabled while marking it as a candidate for replacement by policyengine-observability.
• Add granular runtime timing segments, nested segment_tree assertions, handled error recording, and direct Logfire legacy helper tests.
• Update the simulation API lockfile to resolve policyengine-observability v1.3.0.

Validation

• uv run pytest tests/test_simulation_output_builder.py tests/test_logfire_legacy.py tests/gateway/test_endpoints.py tests/test_modal_bundle_image.py tests/test_observability.py -q passed: 86 tests.
• uv run ruff format --check fixtures/gateway/shared.py src/modal/app.py src/modal/budget_window_batch.py src/modal/budget_window_scheduler.py src/modal/gateway/app.py src/modal/gateway/auth.py src/modal/gateway/endpoints.py src/modal/gateway/errors.py src/modal/logfire_legacy.py src/modal/logging_redaction.py src/policyengine_api_simulation/main.py src/policyengine_api_simulation/observability.py src/policyengine_api_simulation/simulation_output_builder.py src/policyengine_api_simulation/simulation_runtime.py tests/gateway/test_endpoints.py tests/gateway/test_errors.py tests/test_logfire_legacy.py tests/test_modal_bundle_image.py tests/test_observability.py tests/test_simulation_output_builder.py passed.
• git diff --check passed.
• uv run ruff format --check . still reports pre-existing formatting drift outside this branch in tests/test_dataset_uri.py, tests/test_hf_dataset.py, and tests/test_standalone_simulation_contract.py.