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 -->

pdm.lock

@@ -5,7 +5,7 @@
 groups = ["default", "dev"]
 strategy = ["inherit_metadata"]
 lock_version = "4.5.0"
-content_hash = "sha256:db686d496c2229b3bd3d33e5cddb92024bb15b0641c8cad58af92e97bab507b8"
+content_hash = "sha256:2786f3461151a94a8f76ed4ca2b2f3d8d7242d0004e17f1e1ae326b0d6529acd"
 
 [[metadata.targets]]
 requires_python = ">=3.10,<3.15"
@@ -553,6 +553,17 @@ files = [
     {file = "exceptiongroup-1.3.0.tar.gz", hash = "sha256:b241f5885f560bc56a59ee63ca4c6a8bfa46ae4ad651af316d4e81817bb9fd88"},
 ]
 
+[[package]]
+name = "execnet"
+version = "2.1.1"
+requires_python = ">=3.8"
+summary = "execnet: rapid multi-Python deployment"
+groups = ["dev"]
+files = [
+    {file = "execnet-2.1.1-py3-none-any.whl", hash = "sha256:26dee51f1b80cebd6d0ca8e74dd8745419761d3bef34163928cbebbdc4749fdc"},
+    {file = "execnet-2.1.1.tar.gz", hash = "sha256:5189b52c6121c24feae288166ab41b32549c7e2348652736540b9e6e7d4e72e3"},
+]
+
 [[package]]
 name = "filelock"
 version = "3.20.0"
@@ -1360,6 +1371,21 @@ files = [
     {file = "pytest_rerunfailures-16.1.tar.gz", hash = "sha256:c38b266db8a808953ebd71ac25c381cb1981a78ff9340a14bcb9f1b9bff1899e"},
 ]
 
+[[package]]
+name = "pytest-xdist"
+version = "3.8.0"
+requires_python = ">=3.9"
+summary = "pytest xdist plugin for distributed testing, most importantly across multiple CPUs"
+groups = ["dev"]
+dependencies = [
+    "execnet>=2.1",
+    "pytest>=7.0.0",
+]
+files = [
+    {file = "pytest_xdist-3.8.0-py3-none-any.whl", hash = "sha256:202ca578cfeb7370784a8c33d6d05bc6e13b4f25b5053c30a152269fd10f0b88"},
+    {file = "pytest_xdist-3.8.0.tar.gz", hash = "sha256:7e578125ec9bc6050861aa93f2d59f1d8d085595d6551c2c90b6f4fad8d3a9f1"},
+]
+
 [[package]]
 name = "pywin32-ctypes"
 version = "0.2.3"