Fix operator API health leak, preserve bootstrap break‑glass, and fix structured logging

[aidankhogg]

Motivation

• Prevent unauthenticated health responses from leaking sensitive last_error details while still indicating an error occurred.
• Preserve local lifecycle automation by allowing an explicitly configured X-Bootstrap-Secret to act as a break‑glass admin credential even after Phase 4 (OIDC) completion.
• Eliminate Loguru formatting errors caused by nested extra payloads in the HTTP request middleware.

Description

• Stop returning raw last_error in the unauthenticated health response and keep only the presence flag via last_error_present in netengine/api/routes.py.
• Accept an explicitly configured X-Bootstrap-Secret as a local admin credential before falling back to normal Phase 4/OIDC behavior in netengine/api/auth.py.
• Change the structured request logging in netengine/logs/middleware.py to bind event metadata rather than passing it under extra, avoiding Loguru format_map KeyError issues.
• Update tests to reflect the promoted experimental gateway feature-state behavior and mirrored gateway validation requirements (tests/test_cli.py, tests/test_gateway_federation.py, tests/test_spec_parsing.py).

Testing

• Ran the full test suite with pytest -q, producing 573 passed, 10 skipped, 24 warnings.
• Re-ran targeted failing tests to verify fixes: the gateway/spec parsing tests and operator API health/reload tests passed.
• Confirmed structured logging no longer triggers Loguru format errors during request handling in the test runs.

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Codex Task