feat: speed up and stabilise test suite (#676)

## ℹ️ Description
*Provide a concise summary of the changes introduced in this pull
request.*

- Link to the related issue(s): Issue #
- Describe the motivation and context for this change.

Refactors the test harness for faster and more reliable feedback: adds
deterministic time freezing for update checks, accelerates and refactors
smoke tests to run in-process, defaults pytest to xdist with durations
tracking, and adjusts CI triggers so PRs run the test matrix only once.

## 📋 Changes Summary

- add pytest-xdist + durations reporting defaults, force deterministic
locale and slow markers, and document the workflow adjustments
- run smoke tests in-process (no subprocess churn), mock update
checks/logging, and mark slow specs appropriately
- deflake update check interval tests by freezing datetime and simplify
FixedDateTime helper
- limit GitHub Actions `push` trigger to `main` so feature branches rely
on the single pull_request run

### ⚙️ Type of Change
Select the type(s) of change(s) included in this pull request:
- [ ] 🐞 Bug fix (non-breaking change which fixes an issue)
- [x] ✨ New feature (adds new functionality without breaking existing
usage)
- [ ] 💥 Breaking change (changes that might break existing user setups,
scripts, or configurations)


## ✅ Checklist
Before requesting a review, confirm the following:
- [x] I have reviewed my changes to ensure they meet the project's
standards.
- [x] I have tested my changes and ensured that all tests pass (`pdm run
test`).
- [x] I have formatted the code (`pdm run format`).
- [x] I have verified that linting passes (`pdm run lint`).
- [x] I have updated documentation where necessary.

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Tests**
* Ensure tests run in a consistent English locale and restore prior
locale after each run
  * Mark integration scraping tests as slow for clearer categorization
* Replace subprocess-based CLI tests with an in-process runner that
returns structured results and captures combined stdout/stderr/logs;
disable update checks during smoke tests
* Freeze current time in update-check tests for deterministic assertions
* Add mock for process enumeration in web‑scraping unit tests to
stabilize macOS-specific warnings

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.github/workflows/build.yml

@@ -10,9 +10,8 @@ on:  # https://docs.github.com/en/actions/reference/workflows-and-actions/events
     # https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#schedule
     - cron: '0 15 1 * *'
   push:
-    branches-ignore:  # build all branches except:
-    - 'dependabot/**'     # prevent GHA triggered twice (once for commit to the branch and once for opening/syncing the PR)
-    - 'dependencies/pdm'  # prevent GHA triggered twice (once for commit to the branch and once for opening/syncing the PR)
+    branches:
+    - main
     tags-ignore:  # don't build tags
     - '**'
     paths-ignore:
@@ -55,6 +54,8 @@ jobs:
   build:
   ###########################################################
 
+    if: github.event_name != 'push' || github.ref == 'refs/heads/main'
+
     permissions:
       packages: write