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 -->

tests/conftest.py

@@ -15,6 +15,7 @@ Fixture Organization:
 - Test data fixtures: Shared test data (description_test_cases)
 """
 import os
+from collections.abc import Iterator
 from typing import Any, Final, cast
 from unittest.mock import MagicMock
 
@@ -23,7 +24,7 @@ import pytest
 from kleinanzeigen_bot import KleinanzeigenBot
 from kleinanzeigen_bot.model.ad_model import Ad
 from kleinanzeigen_bot.model.config_model import Config
-from kleinanzeigen_bot.utils import loggers
+from kleinanzeigen_bot.utils import i18n, loggers
 from kleinanzeigen_bot.utils.web_scraping_mixin import Browser
 
 loggers.configure_console_logging()
@@ -198,6 +199,15 @@ def silence_nodriver_logs() -> None:
     loggers.get_logger("nodriver").setLevel(loggers.WARNING)
 
 
+@pytest.fixture(autouse = True)
+def force_english_locale() -> Iterator[None]:
+    """Ensure tests run with a deterministic English locale."""
+    previous_locale = i18n.get_current_locale()
+    i18n.set_current_locale(i18n.Locale("en", "US", "UTF-8"))
+    yield
+    i18n.set_current_locale(previous_locale)
+
+
 # ============================================================================
 # Smoke Test Fixtures - Special fixtures for smoke tests
 # ============================================================================