Split GitHub workflow contract tests below the Pylint module limit

[codeforester]

Goal

Split the GitHub workflow contract tests into focused modules so the Pylint module-size gate passes without weakening the limit.

Background

tests/test_github_workflows.py is 1002 lines on main, which fails Pylint's default too-many-lines limit of 1000. The largest cohesive section is the issue-branch-policy contract and scenario suite. This is follow-up maintenance after the recent workflow-hardening changes; it is independent of the merged documentation change in #2206.

Scope

• Move the issue-branch-policy workflow contract and scenario tests into a focused test module.
• Keep shared process/YAML helpers reusable and preserve test behavior.
• Update contract runners, lightweight CI commands, documentation, and hardening-test references that name the existing module.
• Keep the general workflow contract tests in tests/test_github_workflows.py.

Acceptance Criteria

• No module checked by Pylint exceeds the configured/default 1000-line limit.
• The moved issue-branch-policy tests run and retain their existing assertions.
• Contract runners and lightweight CI invoke the complete workflow contract coverage.
• The contract registry and enforcement references remain accurate.
• No Pylint threshold increase or blanket too-many-lines suppression is introduced.

Validation

• python -m pytest tests/test_github_workflows.py tests/test_issue_branch_policy_workflow.py -q
• bash tests/contracts/run.sh or the focused contract checks it composes.
• git diff --check
• Pylint across the supported Python matrix.

Non-goals

• Do not change workflow behavior or issue-branch-policy semantics.
• Do not change the Pylint policy to accommodate this file.
• Do not combine unrelated workflow or documentation changes.