ci: skip SDK test matrices on markdown-only changes

[opieter-aws]

Description

Don't run the entire integ test suite on markdown changes only. See e.g. #2959

Verified all test suites run as expected when .md changes are combined with source code changes.

Related Issues

Motivated by the CI behavior observed on #2959.

Documentation PR

Not applicable.

Type of Change

Other (please describe): CI configuration — refine merge-gate path filtering.

Testing

Cannot run the merge gate locally, but I traced the edited filters against the real paths-filter matching algorithm (per-pattern picomatch({dot:true}), OR-ed under the some quantifier) for representative change-sets:

• strands-py/AGENTS.md only → python does not fire (intended)
• strands-py/src/strands/agent/agent.py → python fires (intended)
• strands-py/AGENTS.md + …/agent.py together → python fires — the critical mixed case: any non-.md file in the changeset still triggers the suite
• strands-ts/AGENTS.md only → typescript does not fire (intended)
• site/** → docs fires, SDK suites do not (unchanged)

Note: because every filter lists .github/workflows/ci.yml, this PR itself will (correctly) trigger all three suites.

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Assessment: Approve

Focused, correct CI change. I reproduced the paths-filter matching behavior locally with picomatch (both v2.3.2 and v4.0.4) under {dot: true} and confirmed the extglob does exactly what the PR claims — including the critical mixed-changeset case.

What I verified

• strands-py/**/!(*.md) excludes .md at every depth (AGENTS.md, README.md, docs/*.md, a/b/c.md).
• Non-.md files match at every depth, including no-extension (Makefile) and dotfiles (.gitignore).
• Top-level non-.md with zero intermediate dirs (e.g. conftest.py) still matches — so a mixed .md + source changeset correctly fires the suite.
• The inline comment's warning about a bare !**/*.md negation is accurate and worth keeping.
• The python-check-markdown-links workflow is schedule/workflow_dispatch-triggered, not gated by ci.yml, so markdown link checking is unaffected by this change.

One minor, non-blocking consistency nit left inline (strandly/**). Nice, well-reasoned change with the manual verification documented in the description.