Add Python version matrix testing to CI

[Copilot]

CI was testing against a single Python version. This adds matrix testing across Python 3.11, 3.12, and 3.13 to catch version-specific regressions.

Changes

• .github/workflows/create_test_conda_env.yml
  • Added strategy.matrix over python-version: ["3.11", "3.12", "3.13"] with fail-fast: false so all versions run to completion
  • Replaced environment-file: environment.yaml with python-version: ${{ matrix.python-version }} in the setup-miniconda step — the env file hardcodes a Python version and can't be parameterized
  • Added channels: conda-forge,noaa-gfdl directly to the setup step (consistent with build_conda.yml)
  • Added explicit "Install dependencies" step mirroring environment.yaml packages, decoupled from the env file so each matrix job resolves deps against the correct Python version

strategy:
  matrix:
    python-version: ["3.11", "3.12", "3.13"]
  fail-fast: false

Issue ticket number and link (if applicable)

Checklist before requesting a review

• I ran my code
• I tried to make my code readable
• I tried to comment my code
• I wrote a new test, if applicable
• I wrote new instructions/documentation, if applicable
• I ran pytest and inspected it's output
• I ran pylint and attempted to implement some of it's feedback
• No print statements; all user-facing info uses logging module

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