Fix many-to-many embedCount() when multiple relations are eager loaded

[techmahedy]

Summary

This PR fixes wrong relationship_count values when embed() is used with multiple relations.

Problem

When category, user, and tags were eager-loaded together, the many-to-many count logic used stale relation key state and could return incorrect counts.

Solution

The framework now uses the model primary key directly for many-to-many count grouping instead of relying on mutable relation state.

Changes

• Fixed many-to-many count key resolution in Builder.php
• Removed the stale $localKey dependency in count handling

Verification

• Confirmed Post::embed(['category:name', 'user:name', 'tags:name'])->embedCount('tags') returns correct counts