Fix deprecated max_size parameter handling in DETR image processors
Author: nck90
Pull request: yes

Fixes #37939

The deprecated max_size parameter was incorrectly overriding the size parameter completely instead of being properly handled. This affected both ConditionalDetrImageProcessor and DetrImageProcessor.

Changes:
- Fixed __init__ method to properly handle max_size parameter by setting longest_edge when size dict doesn't have it, instead of replacing size
- Fixed preprocess method to handle max_size parameter correctly
- Added max_size as explicit parameter to __init__ for proper from_dict support
- Added comprehensive tests for max_size parameter handling

The fix ensures backward compatibility while properly deprecating the max_size parameter in favor of size['longest_edge'].

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Comments:
cc @qubvel @yonigozlan 

Hello @nck90, thanks for opening a PR, but unfortunately, we can't remove support for `max_size` for DETR because some models on the hub have not yet updated their configs.