Generate WindowEvent-to-EventSlot dictionary in window events generator

[zxyao145]

Motivation

• Replace individual generated backing fields per window event with a single lookup to simplify generated code and reduce duplication.
• Support both single-argument and two-argument window event handler signatures via a common dictionary-backed slot abstraction.
• Make generated add/remove accessors and JS-invokable callbacks resolve slots at runtime to centralize slot storage and casting.

Description

• Updated the source generator WindowEventsGenerator to emit a single private readonly Dictionary<WindowEvent, EventSlot> _eventSlots and populate it with EventSlot<...> instances for each discovered event instead of generating one field per event (src/BQuery.SourceGenerators/WindowEventsGenerator.cs).
• Changed generated event accessors and [JSInvokable] callback code to resolve the correct slot from _eventSlots and cast to the appropriate EventSlot<...> before adding/removing handlers or invoking (src/BQuery.SourceGenerators/WindowEventsGenerator.cs).
• Extended BqEvents to include a non-generic EventSlot base type and a new EventSlot<T1, T2> implementation alongside existing EventSlot<T> so dictionary values can represent both one-arg and two-arg event signatures (src/BQuery/BqEvents.cs).
• Removed the previous per-event SlotName generation and switched to dictionary-keyed slot lookup in generated output (src/BQuery.SourceGenerators/WindowEventsGenerator.cs and src/BQuery/BqEvents.cs).

Testing

• Ran git diff --check as a quick validation of working-tree changes and it succeeded.
• Ran repository inspections (git status, file diffs) to verify the intended changes were staged and present.
• Attempted to run dotnet build to validate the generator in a real build but it could not be executed in this environment because dotnet is not installed (bash: command not found: dotnet).

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[chatgpt-codex-connector]

Prevent duplicate WindowEvent keys in generated slot map

Generating _eventSlots with WindowEvent keys causes a runtime ArgumentException when two declared events compare equal, which already happens in this repo (OnDoubleClick and OnClick both have name "click", and WindowEvent.Equals compares by Name case-insensitively). Because the dictionary is initialized in the BqEvents instance initializer, constructing BqEvents will fail before any handlers can be used.

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