Add golden fixtures and contract tests for structured route logs

[Disaster-Terminator]

Motivation

• Freeze the structured route log shape with a stable golden fixture so downstream observability work can rely on a fixed contract.
• Make log-shape drift obvious in CI without including prompts, secrets, Authorization headers, or raw request bodies.
• Ensure existing summary and budgeting tooling (router_log_summary.py and check_route_error_budget.py) handle mixed records (explicit route_id, null/passthrough, and missing route_id) and malformed trailing JSON gracefully.

Description

• Add a golden NDJSON fixture at tests/samples/router_logs_contract.ndjson that includes route_complete, route_error, a passthrough (route_id: null), a legacy log missing route_id, a startup non-route line, and a trailing malformed JSON line.
• Add contract tests to tests/test_router_log_summary.py that parse the fixture with parse_route_records and assert totals, routes vs targets buckets, error_types, and parse diagnostics, and a safety test that asserts forbidden sensitive patterns are not present in the fixture.
• Add a contract test to tests/test_check_route_error_budget.py that exercises check_budget on the fixture with permissive BudgetConfig and asserts target_error_rates, route_error_rates (including unknown for missing route IDs), error_types, and parse diagnostics.
• Keep this as a test/contract layer only and do not change routing logic or introduce any secrets or production logs.

Testing

• Ran uv run python -m pytest -q and the test suite passed (153 passed, 1 warning).
• Ran uv run python scripts/router_log_summary.py --json < tests/samples/router_logs_contract.ndjson which produced a JSON summary successfully showing expected counts and diagnostics.
• Ran uv run python scripts/check_route_error_budget.py < tests/samples/router_logs_contract.ndjson which executed the CLI reading from stdin and printed a failing budget result (non-zero exit) because the fixture intentionally contains route errors; the fixture-driven unit test for check_budget uses a permissive BudgetConfig and passed under pytest.

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Codex Task

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