[ENH] V1 → V2 API Migration - core structure

[geetu040]

Towards #1575

This PR sets up the core folder and file structure along with base scaffolding for the API v1 → v2 migration.

It includes:

• Skeleton for the HTTP client, backend, and API context
• Abstract resource interfaces and versioned stubs (*V1, *V2)
• Minimal wiring to allow future version switching and fallback support

No functional endpoints are migrated yet. This PR establishes a stable foundation for subsequent migration and refactor work.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.45887% with 236 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.60%. Comparing base (7feb2a3) to head (76b92bb).

Files with missing lines	Patch %	Lines
openml/_config.py	72.63%	78 Missing ⚠️
openml/_api/clients/http.py	69.64%	68 Missing ⚠️
openml/_api/setup/builder.py	29.03%	22 Missing ⚠️
openml/_api/setup/backend.py	66.66%	16 Missing ⚠️
openml/_api/resources/base/versions.py	83.11%	13 Missing ⚠️
openml/_api/resources/base/base.py	71.42%	10 Missing ⚠️
openml/cli.py	0.00%	8 Missing ⚠️
openml/_api/resources/base/fallback.py	84.21%	6 Missing ⚠️
openml/runs/functions.py	0.00%	5 Missing ⚠️
openml/_api_calls.py	70.00%	3 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1576      +/-   ##
==========================================
- Coverage   52.82%   48.60%   -4.22%     
==========================================
  Files          37       61      +24     
  Lines        4371     5022     +651     
==========================================
+ Hits         2309     2441     +132     
- Misses       2062     2581     +519     

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[geetu040]

Notes for stacked PRs on updating tests

Please follow tests/test_api/test_versions.py to write tests. I have created fixtures in conftest.py, you can use them.

1. You don't need to create dummy resource, you will import relevant resource from openml._api
2. create 2 fixtures [resource]_v1 and [resource]_v2 using the clients
3. write v1-specific tests, v2-specific tests and tests to match outputs of v1 and v2 where applicable
4. you can choose between mocking or live-server call. I would prefer that delete and post methods are mocked since they are more prune to race conditions.
5. If you want to access a config, access it directly from openml.config.[var] instead of http_client.[var] or [resource].[var]

FYI: @EmanAbdelhaleem @JATAYU000 @rohansen856 @satvshr @Omswastik-11