Implement #7718 (Exposing hitboxes and their bounds)

[yuberee]

Added GetHitboxes and RemoveHitbox to ModelHitboxes
Expose Hitbox's bounds
Thank you

[yuberee]

• Added summaries
• Added GetHitbox with overloads for:
  • Bone index
  • Bone name
  • BoneCollection.Bone
  • PhysicsBody
• Added HitboxesRebuilt action to subscribe to

Having model hitboxes be this accessible allows us to easily target and modify specific ones such as increasing the size of the head hitbox on lower game difficulties or removing hitboxes entirely on "dismembered" enemies.
My other very specific use case was allowing bots to target hitboxes such as the chest and pelvis if they had shotguns.

[aylaylay]

This seems mostly fine to me. I don't like the fact you can add and remove hitboxes but AddHitbox is already public here but that may have been a mistake.

[Copilot]

Pull request overview

This PR implements issue #7718 by exposing hitboxes and their bounds to the public API. It adds methods to retrieve and manipulate hitboxes in ModelHitboxes, exposes the Bounds property on Hitbox, and makes the ModelChanged event public on ModelRenderer.

Changes:

• Added methods to query hitboxes by bone index, bone, bone name, and physics body
• Exposed the Bounds property on Hitbox from internal to public
• Made ModelChanged event on ModelRenderer public with documentation
• Added HitboxesRebuilt event to notify when hitboxes are rebuilt

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
engine/Sandbox.Engine/Scene/Components/Render/ModelRenderer.cs	Made ModelChanged event public and added XML documentation
engine/Sandbox.Engine/Scene/Components/Game/ModelHitboxes.cs	Added GetHitboxes, RemoveHitbox, and overloaded GetHitbox methods; added HitboxesRebuilt event; refactored properties to use field keyword
engine/Sandbox.Engine/Scene/Components/Game/Hitbox.cs	Changed Bounds property visibility from internal to public

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