sklearn: add sagemaker e2e tests

[jinyan-li1]

Summary

Adds native SageMaker E2E tests for the sklearn DLC (follow-up to #6344). Wires them into the pipeline as integ-test-full, gated on inputs.release=true so they only run on release flows (matches xgboost pattern).

• test/sklearn/e2e/*.py — 5 test modules (inference, MME + single-model, network isolation, script mode, inference pipelines)
• test/sklearn/conftest.py + test/sklearn/e2e/conftest.py — helpers mirror test/xgboost/e2e/ shape
• .github/workflows/sklearn.tests-integ.yml — reusable E2E workflow with 5-module matrix
• .github/workflows/sklearn.pipeline.yml — adds integ-test-full job + wires into release-gate

Depends on #6360 (adds the workflow_dispatch entry point needed to trigger this pipeline manually).

Test plan

• Dispatch "Dispatch - Scikit-learn Release" workflow against this branch (after sklearn: add dispatch-release workflow for manual pipeline runs #6360 merges) to validate full pipeline with `release: true`
• Confirm all 5 E2E test modules pass against a DLC-built candidate image