fix: Coordinator split, Gemini chat context, review findings fixes

- Extract HistoryManager (history.py) and MetricsManager (metrics.py) from coordinator
- Fix Gemini chat: use client.chats.create(history=...) instead of sequential send_message
- Fix float("inf") in metrics breaking json.dump persistence
- Fix _get_current_state checking wrong error key ("error" vs "error_message")
- Fix API key re-entry requirement when endpoint/provider changes in options flow
- Make CONF_API_KEY optional in options schema (stored key used as fallback)
- Add DNS rebinding protection via socket.getaddrinfo in validate_endpoint
- Remove mass assignment vulnerability in config_flow._create_entry
- Add description_placeholders to all error re-show paths
- Fix history migration overwriting existing JSON data
- Return list copy from get_limited_history to prevent reference leaks
- Add history_info to _get_safe_initial_state for data shape consistency
- Add defensive None check in _get_sanitized_last_response
- Move dt_util import to module level in config_flow
- Remove dead fallback branches in coordinator.last_response property
- Fix sensor min_latency display after float("inf") removal
- Fix history directory permissions from 0o777 to 0o755
- Deduplicate schema definitions via _build_provider_schema()
- Use centralized build_auth_headers from providers.py

custom_components/ha_text_ai/sensor.py

@@ -9,8 +9,6 @@ Sensor platform for HA Text AI.
 import logging
 import math
 from typing import Any, Dict
-from datetime import datetime, timedelta
-
 from homeassistant.components.sensor import (
     SensorEntity,
     SensorEntityDescription,
@@ -277,9 +275,7 @@ class HATextAISensor(CoordinatorEntity, SensorEntity):
                     METRIC_FAILED_REQUESTS: metrics.get("failed_requests", 0),
                     METRIC_AVERAGE_LATENCY: round(metrics.get("average_latency", 0), 2),
                     METRIC_MAX_LATENCY: round(metrics.get("max_latency", 0), 2),
-                    METRIC_MIN_LATENCY: (metrics.get("min_latency")
-                                       if metrics.get("min_latency") != float("inf")
-                                       else None),
+                    METRIC_MIN_LATENCY: metrics.get("min_latency", 0) or None,
                 })
 
             # Last response handling