fix!: migrate from httpx to httpx2

[kjvbrt]

Summary

• httpx is dormant; pydantic has taken over maintenance under the httpx2 name (https://github.com/pydantic/httpx2)
• Swap the httpx dependency for httpx2 and update all imports/usages (httpx.Client, httpx.AsyncClient, httpx.Response, httpx.HTTPError, etc.) across src/, tests/, and benchmark_tests/

Breaking change

• Renamed the Client/AsyncClient constructor kwarg httpx_max_connections to max_connections.

Closes #46

[chrisburr]

I would go with max_connections.

[kjvbrt]

Should max_connections be used also in the DiracX proper?

[github-actions]

Test	3.11	3.12	3.13	3.14
copy_object_aio	1.044	1.049	1.697	1.116
copy_object_sync	1.297	1.367	1.396	1.391
create_bucket_aio	0.987	0.990	0.986	1.058
create_bucket_aio_cm	0.995	1.000	0.989	1.048
create_bucket_sync	1.154	1.166	1.167	1.121
create_bucket_sync_cm	1.148	1.166	1.150	1.110
delete_objects_aio	0.977	0.970	1.005	0.775
delete_objects_aio_cm	0.991	1.003	0.992	1.026
delete_objects_sync	1.008	1.025	1.005	0.897
delete_objects_sync_cm	1.018	1.015	1.010	1.048
generate_presigned_post_aio	9.196	8.869	8.950	8.963
generate_presigned_post_aio_cm	9.337	9.024	8.945	9.170
generate_presigned_post_sync	7.291	6.969	7.124	7.226
generate_presigned_post_sync_cm	7.390	6.902	7.055	6.907
generate_presigned_url_aio	15.765	15.270	15.675	18.379
generate_presigned_url_aio_cm	15.958	15.525	15.389	17.447
generate_presigned_url_sync	15.645	14.374	15.155	16.659
generate_presigned_url_sync_cm	15.987	14.809	14.214	17.415
head_bucket_aio	0.991	1.003	0.995	1.032
head_bucket_aio_cm	1.012	0.986	1.003	1.007
head_bucket_sync	1.462	1.565	1.562	1.645
head_bucket_sync_cm	1.469	1.580	1.557	1.561
head_object_aio	0.917	0.948	0.933	0.946
head_object_aio_cm	0.920	0.944	0.908	0.945
head_object_sync	1.522	1.578	1.580	1.526
head_object_sync_cm	1.538	1.604	1.561	1.544
list_objects_aio	1.122	1.142	1.141	1.137
list_objects_sync	1.316	1.346	1.296	1.409
put_object_aio	1.186	1.170	1.129	1.140
put_object_sync	0.923	1.653	1.594	1.673
upload_file_aio	1.106	1.086	1.064	1.124
upload_file_sync	2.088	2.147	2.103	2.259

[aldbr]

Should max_connections be used also in the DiracX proper?

What's going to happen is that once this PR is merged, renovate will try to update the diracx lock file in DIRACGrid/diracx#898.

At this point, we will need to make the same adjustment in diracx as part of the renovate PR I would say

[aldbr]

To inform consumers we are introducing a breaking change, we should use the ! in the PR title. See: https://www.conventionalcommits.org/en/v1.0.0/#commit-message-with--to-draw-attention-to-breaking-change

[aldbr]

Note: while reviewing the PR, I noticed a few possible changes that could be made but they are outside of the scope of the issue. So I created #49 and #50