Pin RabbitMQ broker image to 3.x in Celery tests

[CasperWA]

Summary

Pin the RabbitMQ Docker image used by the test_celery_remote tests to rabbitmq:3 instead of rabbitmq:latest.
RabbitMQ 4.x removed support for the transient_nonexcl_queues feature, which Celery/kombu requires during worker startup, causing all Linux CI jobs to fail.

Motivation

Fixes #680

The pytest-celery library defaults to rabbitmq:latest for its broker container.
A recent promotion of RabbitMQ 4.x to that tag broke the test_celery_remote test on every Linux runner, blocking every incoming PR (e.g. #677).

Changes

tests/strategies/transformation/conftest.py (new file)

Override the default_rabbitmq_broker_image fixture — the extension point that pytest-celery explicitly exposes for this purpose — to return "rabbitmq:3".
No other test behaviour changes.

Test Plan

• pre-commit run -a passes with no modifications
• test_celery_remote passes on Linux runners (verified by CI on this PR)

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[Copilot]

Pull request overview

Pins the RabbitMQ Docker image used by the test_celery_remote integration test setup to RabbitMQ 3.x to prevent CI failures caused by RabbitMQ 4.x removing/locking down the transient_nonexcl_queues behavior required by Celery/kombu during worker startup.

Changes:

• Add a local default_rabbitmq_broker_image fixture override for pytest-celery.
• Return "rabbitmq:3" instead of relying on rabbitmq:latest.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.44%. Comparing base (bc2987e) to head (73354b4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #681   +/-   ##
=======================================
  Coverage   83.44%   83.44%           
=======================================
  Files          14       14           
  Lines         610      610           
=======================================
  Hits          509      509           
  Misses        101      101           

Flag	Coverage Δ
linux	83.44% <ø> (ø)
linux-strategies	83.44% <ø> (ø)
windows	82.78% <ø> (ø)
windows-strategies	82.78% <ø> (ø)

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[CasperWA]

Squash commit message:

Title

Pin RabbitMQ broker image to 3.x in Celery tests (#680)

Body

RabbitMQ 4.x removed transient_nonexcl_queues, which Celery/kombu
uses for the mingle boot-step. Pin to rabbitmq:3 via the
default_rabbitmq_broker_image fixture override provided by
pytest-celery for exactly this purpose.