Fix _bulk_create for ManyToManyField with custom through models (#477)

[amureki]

Describe the change
I figured that bulk_create was using incorrect properties (the related model's name/related_name) for getting FK names.

Replaced field.remote_field.name and field.related_model._meta.model_name with Django's field.m2m_field_name() and field.m2m_reverse_field_name() (ref) which return the correct FK field names for both auto-created and custom through models.

PR Checklist

• Change is covered with tests
• CHANGELOG.md is updated if needed

Summary by CodeRabbit

• Bug Fixes

  • Fixed bulk creation of objects with many-to-many relationships when using custom through models.

• Tests

  • Added regression tests for bulk creation with many-to-many relationships and through models.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Fixed bulk_create to properly handle ManyToManyField relationships with custom through models by switching from using remote field names and model meta names to using M2M helper methods field.m2m_field_name() and field.m2m_reverse_field_name() for correct key instantiation.

Changes

Cohort / File(s)	Summary
Bug fix model_bakery/baker.py	Modified through-model field value construction in bulk_create to use M2M helper methods instead of manually constructed field names for M2M relationships
Test coverage tests/test_baker.py	Added two regression tests: test_bulk_create_with_m2m and test_bulk_create_with_m2m_through_model validating bulk creation of M2M relationships with and without custom through models
Documentation CHANGELOG.md	Added changelog entry documenting the M2M bulk_create fix for custom through models

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 Through fields we hop with names so right,
M2M relations, shining bright!
From manual paths to helper ways,
Bulk create works in modern days! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: fixing _bulk_create for ManyToManyField with custom through models, which is directly reflected in the code changes to model_bakery/baker.py.

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✨ Finishing touches

• 📝 Generate docstrings

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