Fix LDM3D NumPy batch preprocessing

[rathodkunj2005]

What does this PR do?

Fixes #14428.

VaeImageProcessorLDM3D.preprocess supports NumPy RGB/depth batches, but the depth branch was choosing between concatenate and stack by checking the already-processed RGB tensor instead of the original depth input. That made valid batched depth arrays become 5D and fail in numpy_to_pt with ValueError: axes don't match array.

This PR:

• forwards the LDM3D image processor init arguments to VaeImageProcessor.__init__, matching the documented/configured behavior;
• checks depth[0].ndim when batching NumPy depth inputs;
• adds regression coverage for LDM3D config propagation and batched NumPy RGB/depth preprocessing.

Verification:

$ python -m pytest tests/others/test_image_processor.py -k 'ldm3d' -q
..                                                                       [100%]
2 passed, 10 deselected in 0.07s

$ python -m pytest tests/others/test_image_processor.py -q
............                                                             [100%]
12 passed, 2 warnings in 0.01s

$ python -m ruff check src/diffusers/image_processor.py tests/others/test_image_processor.py
All checks passed!

AI disclosure / self-review notes:

• I used an AI agent to help identify, implement, and verify this focused fix, and reviewed the final diff before submission.
• Blocking issues found in self-review: none.
• Non-blocking issues found in self-review: none.
• Dead-code advisory: none; the changed branches are exercised by the added tests.

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