fix(runtime): apply rate-limit and auto-mode config changes immediately

[alexk-dev]

Summary

• fix TokenBucketRateLimiter so existing buckets are recreated when runtime limits change, ensuring new rate-limit values apply immediately for global, channel, and llm scopes
• add regression tests that reproduce and protect immediate-apply behavior for updated global, channel, and llm limits
• fix AutoModeScheduler startup and tick behavior so runtime toggling of auto-mode feature is respected without restart: scheduler initializes even when feature is off at boot and each tick re-checks runtime flag
• add regression tests for auto-mode runtime toggle behavior (disabled at startup, disabled at tick time, and disabled->enabled between ticks)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
97.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud