fix(egress): fail closed on poisoned rate state

[cedric013]

Summary

• make the egress rate limiter fail closed when the process-local mutex is poisoned
• add a regression test that poisons the limiter state and verifies no panic/bypass occurs

Tests

• rustfmt --config-path rustfmt.toml --check src/core/egress.rs
• git diff --check -- src/core/egress.rs
• cargo test --lib egress::tests::poisoned_rate_state_fails_closed_without_panicking (attempted locally, cancelled after long compile; CI should exercise it)

Closes #1279