Remove outdated GPU marketplace endpoint and fix staking service logic

- Remove duplicate `/marketplace/gpu/{gpu_id}` endpoint from marketplace_gpu.py
- Remove marketplace_gpu router inclusion from main.py (already included elsewhere)
- Fix staking service staker_count logic to check existing stakes before increment/decrement
- Add minimum stake amount validation (100 AITBC)
- Add proper error handling for stake not found cases
- Fix staking pool update to commit and refresh after modifications
- Update CLI send_transaction to use chain

tests/production/test_production_monitoring.py

@@ -6,8 +6,7 @@ Tests Prometheus metrics, alerting, and SLA monitoring systems
 import pytest
 import requests
 import time
-import json
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Dict, Any
 
 class TestPrometheusMetrics:
@@ -501,10 +500,10 @@ class TestMonitoringIntegration:
         assert uptime_diff < 1.0, f"Uptime difference {uptime_diff} exceeds tolerance of 1.0 second"
         
         # Check timestamps are recent
-        summary_time = datetime.fromisoformat(summary["timestamp"].replace('Z', '+00:00'))
-        system_time = datetime.fromisoformat(system["timestamp"].replace('Z', '+00:00'))
+        summary_time = datetime.fromisoformat(summary["timestamp"].replace('Z', '+00:00')).replace(tzinfo=timezone.utc)
+        system_time = datetime.fromisoformat(system["timestamp"].replace('Z', '+00:00')).replace(tzinfo=timezone.utc)
         
-        now = datetime.utcnow()
+        now = datetime.now(timezone.utc)
         assert (now - summary_time).total_seconds() < 60  # Within last minute
         assert (now - system_time).total_seconds() < 60  # Within last minute