[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 51.51057% with 321 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.40%. Comparing base (8a5532f) to head (65472ed).

Files with missing lines	Patch %	Lines
openml/_api/clients/http.py	23.00%	164 Missing ⚠️
openml/_api/resources/base/versions.py	31.16%	53 Missing ⚠️
openml/_api/resources/base/fallback.py	26.31%	28 Missing ⚠️
openml/_api/setup/builder.py	44.44%	25 Missing ⚠️
openml/_api/setup/backend.py	66.66%	19 Missing ⚠️
openml/_api/resources/base/base.py	57.14%	15 Missing ⚠️
openml/_config.py	80.64%	12 Missing ⚠️
openml/cli.py	0.00%	4 Missing ⚠️
openml/_api/clients/minio.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1576      +/-   ##
==========================================
- Coverage   52.73%   52.40%   -0.34%     
==========================================
  Files          37       61      +24     
  Lines        4399     5038     +639     
==========================================
+ Hits         2320     2640     +320     
- Misses       2079     2398     +319     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[geetu040]

FYI: @JATAYU000 @satvshr @omkar-334 @rohansen856 @EmanAbdelhaleem

Please sync with this PR. The CI tests should now be passing in all the stacked PRs (except some sporadic failures), since now they are using the local docker services created in the CI which provides endpoints for v2-api as well.

You can run these tests locally as well, with the following steps:

Run docker services locally:

git clone --depth 1 https://github.com/openml/services.git
cd ./services        
chmod -R a+rw ./data
chmod -R a+rw ./logs
docker compose --profile rest-api --profile minio --profile evaluation-engine up -d

Run pytest using the local services (add the env var):

OPENML_USE_LOCAL_SERVICES="true" pytest tests/

[PGijsbers]

Presumably they use the same database, is there a reason for using different API keys?