Adds a new public method getGroupById(id) to BatchManager that retrieves a BatchGroup by its numeric ID.

Background

Components such as render, model, element, and sprite store a batchGroupId property, but there was no public API to retrieve the corresponding BatchGroup object from the manager. Users had to either:

• Use getGroupByName() (requires knowing the name)
• Access the private _batchGroups property directly

Changes

• Added getGroupById(id) method that returns the BatchGroup with the specified ID, or null if not found
• Mirrors the existing getGroupByName() API

Public API Additions

/**
 * Retrieves a BatchGroup object with a corresponding id, if it exists, or null otherwise.
 * @param {number} id - The batch group id.
 * @returns {BatchGroup|null}
 */
batchManager.getGroupById(id)

Usage Example

const group = app.batcher.getGroupById(entity.render.batchGroupId);
if (group) {
    console.log(`Group: ${group.name}, dynamic: ${group.dynamic}`);
}

Fixes #8388