Set root to a temporary directory for unit tests

[lstein]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update
• Community Node Submission

Have you discussed this change with the InvokeAI team?

• Yes
• Not needed

Have you updated all relevant documentation?

• Yes
• No

Description

Several unit tests were accessing the real InvokeAI root, leading to non-deterministic behavior. This PR adds a module-level pytest fixture called invokeai_root_dir which returns a path pointing to an initialized (mostly empty) root directory. This is then applied to the test_images unit test.

This PR also fixes an issue in the model manager’s sync_to_dir startup method, which was accessing the real root rather than the test root.

Related Tickets & Documents

• Related Issue #
• Closes #

QA Instructions, Screenshots, Recordings

Before applying this PR, insert an assert False into the first test in tests/app/routers/test_images.py. In the logged failures, you will see messages about the model manager traversing the real root directory. After applying this PR, the messages will show the model manager traversing the temporary root directory.

Merge Plan

Added/updated tests?

• Yes
• No : please replace this line with details on why tests
  have not been included

[optional] Are there any post deployment tasks we need to perform?

[lstein]

This PR is superseded by #5835