[bfs2021]

Problem

pyproject.toml declares azure-functions>=1.21.0 with no upper bound, so a
future 2.x or 3.x release can be selected even though this repository has only
certified the 1.x SDK line. Fixes #646.

Root cause

The requirement and the compatibility policy test both assumed an open bound.
azure-functions 2.x declares Requires-Python >=3.13, so a single unbounded
requirement also lets the resolver pick a major that cannot run on the
interpreters the current wheel and host lanes use.

Fix

• Split [project].dependencies by interpreter, matching the sibling packages:
  >=1.21.0,<2.0.0 on Python < 3.13 and >=1.21.0,<3.0.0 on Python >= 3.13.
  Neither marker permits a 3.x release.
• Point the artifact-python311 and host-smoke "latest" cells at
  azure-functions>=1.21.0,<2.0.0, so those lanes resolve the newest allowed
  1.x SDK instead of being open-ended.
• Extend tests/test_sdk_compatibility_policy.py to assert the two matrix
  entries and to parse both requirements, checking that 3.11 resolves to the
  1.x bound, 3.13 permits 2.x, and no environment can resolve 3.0.0.
• Update docs/sdk-compatibility.md to state the split and to record the gap:
  the 2.x allowance is metadata-only until a wheel-based 2.x lane runs on
  Python 3.13/3.14.

Testing

• pytest -q tests/test_sdk_compatibility_policy.py: 2 passed.
• Both tests fail on the parent commit (the old matrix list and the old
  unbounded requirement), and pass with this change.
• ruff check and ruff format --check on the policy test: clean.
• I did not run the full suite locally: this machine lacks the project's SDK
  and dev extras. make lint typecheck test is left to CI, which provides the
  3.11 and 3.13 interpreters.