fix: reject invalid --ads selector values instead of silent fallback (#834)

## Summary

- When an invalid `--ads` value is explicitly provided (e.g.
`--ads=my-directory-name`), the bot now exits with code 2 and a clear
error message listing valid options, instead of silently falling back to
the command default
- Fixes the numeric ID regex from unanchored `\d+[,\d+]*` (which could
match partial strings like `abc123`) to anchored `^\d+(,\d+)*$`
- Adds `_is_valid_ads_selector()` helper to deduplicate validation logic
across publish/update/download/extend commands

## Motivation

When calling `kleinanzeigen-bot publish --ads=led-grow-light-set`
(passing a directory name instead of a numeric ad ID), the bot silently
fell back to `--ads=due` and republished all due ads — causing
unintended republication of multiple ads and loss of conversations on
those ads.

The silent fallback with only a WARNING log message makes it too easy to
accidentally trigger unwanted operations. An explicit error with exit
code 2 (consistent with other argument validation like
`--workspace-mode`) is the expected behavior for invalid arguments.

## Changes

| File | Change |
|------|--------|
| `src/kleinanzeigen_bot/__init__.py` | Added `_ads_selector_explicit`
flag (set when `--ads` or `--force` is used), `_is_valid_ads_selector()`
helper method, and updated all 4 command blocks
(publish/update/download/extend) to error on explicitly invalid
selectors |
| `resources/translations.de.yaml` | Replaced 3 old fallback messages
with 4 new error messages |
| `tests/unit/test_init.py` | Updated 2 existing tests to expect
`SystemExit(2)` instead of silent fallback, added 2 new tests for
update/extend invalid selectors |

## Test plan

- [x] All 754 unit tests pass (`pdm run utest`)
- [x] Lint clean (`pdm run lint`)
- [x] Translation completeness verified
(`test_all_log_messages_have_translations`,
`test_no_obsolete_translations`)
- [x] `--ads=invalid` on publish/update/download/extend all exit with
code 2
- [x] Default behavior (no `--ads` flag) unchanged for all commands
- [x] Valid selectors (`--ads=all`, `--ads=due`, `--ads=12345,67890`,
`--ads=changed,due`) still work

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

* **Bug Fixes**
* Stricter validation of ad selectors; invalid selectors now terminate
with exit code 2 and preserve safe defaults when no selector is
provided.

* **New Features**
* Support for comma-separated numeric ID lists as a valid selector
format.

* **Tests**
* Unit tests updated to assert non-zero exit on invalid selectors and
verify default-fallback behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Liermann Torsten - Hays <liermann.hays@partner.akdb.de>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/test_init.py

@@ -1152,6 +1152,20 @@ class TestKleinanzeigenBotAdOperations:
             await test_bot.run(["script.py", "download"])
             assert test_bot.ads_selector == "new"
 
+    @pytest.mark.asyncio
+    async def test_run_update_default_selector(self, test_bot:KleinanzeigenBot, mock_config_setup:None) -> None:  # pylint: disable=unused-argument
+        """Test running update command with default selector falls back to changed."""
+        with patch.object(test_bot, "load_ads", return_value = []):
+            await test_bot.run(["script.py", "update"])
+            assert test_bot.ads_selector == "changed"
+
+    @pytest.mark.asyncio
+    async def test_run_extend_default_selector(self, test_bot:KleinanzeigenBot, mock_config_setup:None) -> None:  # pylint: disable=unused-argument
+        """Test running extend command with default selector falls back to all."""
+        with patch.object(test_bot, "load_ads", return_value = []):
+            await test_bot.run(["script.py", "extend"])
+            assert test_bot.ads_selector == "all"
+
     def test_load_ads_no_files(self, test_bot:KleinanzeigenBot) -> None:
         """Test loading ads with no files."""
         test_bot.config.ad_files = ["nonexistent/*.yaml"]
@@ -1172,17 +1186,31 @@ class TestKleinanzeigenBotAdManagement:
 
     @pytest.mark.asyncio
     async def test_run_publish_invalid_selector(self, test_bot:KleinanzeigenBot, mock_config_setup:None) -> None:  # pylint: disable=unused-argument
-        """Test running publish with invalid selector."""
-        with patch.object(test_bot, "load_ads", return_value = []):
+        """Test running publish with invalid selector exits with error."""
+        with pytest.raises(SystemExit) as exc_info:
             await test_bot.run(["script.py", "publish", "--ads=invalid"])
-            assert test_bot.ads_selector == "due"
+        assert exc_info.value.code == 2
 
     @pytest.mark.asyncio
     async def test_run_download_invalid_selector(self, test_bot:KleinanzeigenBot, mock_config_setup:None) -> None:  # pylint: disable=unused-argument
-        """Test running download with invalid selector."""
-        with patch.object(test_bot, "download_ads", new_callable = AsyncMock):
+        """Test running download with invalid selector exits with error."""
+        with pytest.raises(SystemExit) as exc_info:
             await test_bot.run(["script.py", "download", "--ads=invalid"])
-            assert test_bot.ads_selector == "new"
+        assert exc_info.value.code == 2
+
+    @pytest.mark.asyncio
+    async def test_run_update_invalid_selector(self, test_bot:KleinanzeigenBot, mock_config_setup:None) -> None:  # pylint: disable=unused-argument
+        """Test running update with invalid selector exits with error."""
+        with pytest.raises(SystemExit) as exc_info:
+            await test_bot.run(["script.py", "update", "--ads=invalid"])
+        assert exc_info.value.code == 2
+
+    @pytest.mark.asyncio
+    async def test_run_extend_invalid_selector(self, test_bot:KleinanzeigenBot, mock_config_setup:None) -> None:  # pylint: disable=unused-argument
+        """Test running extend with invalid selector exits with error."""
+        with pytest.raises(SystemExit) as exc_info:
+            await test_bot.run(["script.py", "extend", "--ads=invalid"])
+        assert exc_info.value.code == 2
 
 
 class TestKleinanzeigenBotAdConfiguration: