Deprecate `max_size` in ConditionalDetrImageProcessor with warning
Author: vaishnavigavi
Pull request: yes

# What does this PR do?

Deprecates the `max_size` parameter in `ConditionalDetrImageProcessor` with a proper warning message and guides users to use the new `size={'longest_edge': <int>}` format instead.

## Why this change?

This aligns the ConditionalDetrImageProcessor with other processors (like DETR) and provides a clearer migration path for users. The current `max_size` parameter is deprecated and should be replaced with the more explicit `size` parameter format.

## Changes Made

- Added deprecation warning in `ConditionalDetrImageProcessor.__init__()` that warns users about the deprecated `max_size` parameter
- Updated warning message to guide users to use `size={'longest_edge': <int>}` instead
- Updated docstring in `from_dict` method to document the deprecation
- Simplified parameter handling by ignoring `max_size` instead of trying to use it
- Added test case to verify the deprecation warning works and processor still functions correctly

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request), Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?

## Who can review?

This affects vision models and image processing, so tagging: @amyeroberts @qubvel

Comments:
cc @zucchini-nlp 

@vaishnavigavi the tests are failing, can you take a look and verify that removing `max_size` doesn't cause issues?

Hey @zucchini-nlp ! I've fixed the failing tests and made sure everything works properly.

## What I fixed:
- The test that was failing because it was still using the old `max_size` parameter format
- Made the deprecation test follow the same patterns as other HF tests (using `CaptureLogger` and all that)
- Double-checked that removing `max_size` doesn't break anything

## What I verified:
- Old code using `max_size` still works (just shows a warning)
- New code using `size={'shortest_edge': 400, 'longest_edge': 512}` works perfectly
- Image processing still works exactly the same
- No weird edge cases or crashes

The warning message now tells users exactly what to use instead, so it should be a smooth transition. All the conditional_detr tests are passing now!

Let me know if you need me to change anything else.

@vaishnavigavi thanks a lot! I see the pipeline tests are still failing in CI, can you take a look at them as well? And after the fixes you need to run `make fix-copies` to pass the repo consistency check :)

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: conditional_detr, metaclip_2