feat: Weather Assistant Phase 2 - Function calling for weather lookups

[Orinks]

Summary

Add function calling (tool use) to the Weather Assistant so the AI can actively fetch weather data during conversation instead of relying only on pre-loaded context.

Tools to Implement

get_current_weather(location: str)

• Look up current conditions for a given location
• Return temperature, conditions, humidity, wind, etc.
• Use the app's existing weather API client

get_forecast(location: str)

• Get multi-day forecast for a location
• Return period names, temperatures, conditions

get_alerts(location: str)

• Check active weather alerts for a location
• Return alert titles, severity, description

Implementation Notes

• Use OpenAI's function calling / tool use format via OpenRouter
• Functions should use the app's existing WeatherService and API clients
• Handle tool calls in the response loop: call function → send result back → get final response
• Announce final response via Prism (already integrated)
• Graceful error handling if location not found or API fails
• Keep the existing pre-loaded weather context as baseline; tools supplement it

Branch

Work on feature/weather-chat branch.

Key Files

• src/accessiweather/ui/dialogs/weather_assistant_dialog.py — add tool definitions and handling
• src/accessiweather/weather_service.py — existing service to leverage
• src/accessiweather/weather_gov_api_client/ — existing API client

[Orinks]

Implemented in PR #285 (Weather Assistant with function calling, global coverage, fallback chain).