ci(pr): gate builds on changed-files and broaden safe exclusions

[rgsl888prabhu]

Summary

Reduces PR CI time by skipping build jobs when no downstream test or doc consumer needs them, and by excluding more files that never affect builds or tests.

Build gating (`if:`):

• `conda-cpp-build`: `test_cpp ∨ test_python_conda ∨ build_docs`
• `conda-python-build`: `test_python_conda ∨ build_docs`
• All 5 `wheel-build-*`: `test_python_wheels`

New exclusions (applied to all 4 changed-file groups):

• `**/*.md` (docs are `.rst`), `LICENSE`, `.gitattributes`, `.gitignore`, `.clang-format`, `.github/release.yml`
• `ci/thirdparty-testing/**` (only invoked by `nightly.yaml`)
• `.github/workflows/*.yaml` files not reachable from `pr.yaml` (`build.yaml`, `nightly.yaml`, `cloud_ci.yaml`, etc.) — `self_hosted_service_test.yaml` is deliberately kept

Safety

Each group still begins with `'**'` (include everything). `!` lines only remove paths, so any new file that isn't explicitly excluded triggers full CI — fail-closed default preserved.

Expected impact

PR touches only…	Before	After
Markdown / LICENSE / .clang-format	full build+test matrix	`checks` only
`ci/thirdparty-testing/**.sh`	full build+test matrix	`checks` only
Nightly-only workflow files	full build+test matrix	`checks` only
`notebooks/` or `docs//*.rst`	full matrix	skips 5 wheel builds + wheel tests
`cpp/` or `python/`	unchanged	unchanged

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 2f580be0-eab9-4fbc-b5ca-79f46cf4d536

📥 Commits

Reviewing files that changed from the base of the PR and between 248173e and 145817d.

📒 Files selected for processing (1)

• .github/workflows/pr.yaml

🚧 Files skipped from review as they are similar to previous changes (1)

• .github/workflows/pr.yaml

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📝 Walkthrough

Walkthrough

Updated the GitHub Actions PR workflow (.github/workflows/pr.yaml) to expand changed-files exclusion patterns for several test/build groups and to gate downstream build jobs (conda-cpp-build, conda-python-build, wheel-build-*) using changed-files output groups.

Changes

Cohort / File(s)	Summary
Exclusion Pattern Expansions ​.github/workflows/pr.yaml	Expanded changed-files exclude patterns for build_docs, test_cpp, test_python_conda, and test_python_wheels groups (added exclusions like !.clang-format, !.gitattributes, !**/*.md, .github/* workflow/config excludes, ci/thirdparty-testing/**, and adjusted ci/utils/*/plugin/resource-related patterns).
Downstream Job Gating ​.github/workflows/pr.yaml	Added changed-files to needs and introduced if: conditions so conda-cpp-build, conda-python-build, and wheel-build-* jobs run only when corresponding changed_file_groups (test_cpp, test_python_conda, test_python_wheels) are present.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main changes: gating builds based on changed files and broadening file exclusions to skip unnecessary CI jobs.
Description check	✅ Passed	The description clearly explains the PR's purpose, detailing build gating logic, new exclusions, safety mechanisms, and expected impact with concrete examples.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci-pr-workflow-gating

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[jameslamb]

Approving, as I don't think this will cause any correctness problems, but ... I want to mention some reasons why "build unconditionally" is beneficial, in case you haven't considered them:

1. keeps sccache cache fresh (more PRs = faster building for everyone)
2. keeps CI scripts simpler (no need to have checks for whether or not a given run generated artifacts)
3. amortizes the work of detecting and reacting to build issues over time (instead of relying on nightlies to catch them)

I also think it's worth reflecting on the cumulative complexity here... the logic for which CI runs when is getting really complicated in this project (I am thinking about changes like #931 coming too).

If you think the time / resource savings from these changes are worth that complexity and maintenance burden, I won't get in the way.

[rgsl888prabhu]

/merge