CI: test free-threaded Python build

[d-chambers]

Description

Add a gating Ubuntu CPython 3.14 free-threaded build to the main test matrix under the display name free_thread.

• Select the official 3.14t interpreter with actions/setup-python.
• Install DASCore's core, test, and documentation dependencies from free-threaded wheels.
• Assert after importing DASCore that the interpreter was built with Py_GIL_DISABLED=1 and the GIL remains disabled.
• Reuse the existing test-data cache and the normal qmd, test-suite, doctest, and coverage steps.
• Disable matrix fail-fast so a failure in this newer runtime does not cancel conventional Python coverage.

The free-threaded row intentionally does not install dascore[extras]: the current pymseed -> orjson dependency chain rejects free-threaded Python. Optional integration tests therefore skip normally while the supported DASCore core runs as a required job.

Testing

• Python 3.14.6 free-threading runtime assertion: passed.
• Full non-network suite with generated qmd tests: 7,724 passed, 232 skipped, 100 deselected, 2 xfailed.
• Module doctests: 142 passed.
• pre-commit run --all twice: passed, including actionlint.
• Claude CLI ultrareview: one finding addressed by disabling matrix fail-fast.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings and/or appropriate doc page.
• included tests. See testing guidelines.
• added the "ready_for_review" tag once the PR is ready to be reviewed.

Summary by CodeRabbit

• Tests
  • Expanded the automated test matrix to include free-threaded Python 3.14.
  • Added runtime checks to verify the free-threaded runs are executing with the GIL disabled.
  • Adjusted dependency installation and introduced free-threaded-specific test-data caching to improve reliability and speed.
  • Ensured documentation-related tests, unit tests, and doctests run under the appropriate shell/environment for each configuration.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 68a0724a-72bf-4375-8ae2-e8f2aa186829

📥 Commits

Reviewing files that changed from the base of the PR and between 0db4a14 and 306e13a.

📒 Files selected for processing (1)

• .github/workflows/runtests.yml

📝 Walkthrough

Walkthrough

Changes

Free-threaded CI execution

Layer / File(s)	Summary
Matrix and runtime setup .github/workflows/runtests.yml	The test matrix adds Python 3.14t, disables fail-fast, bypasses mamba installation for that entry, installs dedicated dependencies, and verifies GIL-disabled execution.
Free-threaded test-data cache .github/workflows/runtests.yml	The workflow exports cache settings, restores cached test data, primes it on misses, and saves regenerated data.
Matrix test execution .github/workflows/runtests.yml	Documentation and code test steps use the matrix-selected shell, including PYTHON_GIL=0 for free-threaded runs.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the main change: adding free-threaded Python CI testing.
Description check	✅ Passed	The description follows the template with a clear summary, testing notes, and a mostly complete checklist.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch free-thread

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[d-chambers]

@codex review

[chatgpt-codex-connector]

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Reviewed commit: ef443abba7

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.96%. Comparing base (93d2707) to head (306e13a).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #761   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files         163      163           
  Lines       17087    17087           
=======================================
  Hits        17081    17081           
  Misses          6        6           

Flag	Coverage Δ
network	48.76% <ø> (ø)
unittests	99.95% <ø> (ø)

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In @.github/workflows/runtests.yml:
- Around line 139-140: Update the workflow steps running
generate_doc_code_tests.py so shell does not interpolate matrix.test_shell
directly. Move the conditional shell selection into an environment variable and
reference it from shell, or split the steps with if: matrix.free_thread when
different shells are required, ensuring actionlint accepts the configuration.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 32e2cfce-a83d-4de2-86d5-dd71cf71dbe0

📥 Commits

Reviewing files that changed from the base of the PR and between ef443ab and 0db4a14.

📒 Files selected for processing (1)

• .github/workflows/runtests.yml