fix: JSON API Pagination for >25 Ads (#797)

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

- Link to the related issue(s): Closes #789 (completes the fix started
in #793)
- **Motivation**: Fix JSON API pagination for accounts with >25 ads.
Aligns pagination logic with weidi’s approach (starts at page 1), while
hardening error handling and tests. Based on
https://github.com/weidi/kleinanzeigen-bot/pull/1.

## 📋 Changes Summary

- Added pagination helper to fetch all published ads and use it in
delete/extend/publish/update flows
- Added robust handling for malformed JSON payloads and unexpected ads
types (with translated warnings)
- Improved sell_directly extraction with pagination, bounds checks, and
shared coercion helper
- Added/updated tests for pagination and edge cases; updated assertions
to pytest.fail style

### ⚙️ Type of Change
Select the type(s) of change(s) included in this pull request:
- [x] 🐞 Bug fix (non-breaking change which fixes an issue)
- [ ] ✨ 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:cov:unified`).
- [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

* **New Features**
* Reliable multi-page fetching for published ads and buy-now eligibility
checks.

* **Bug Fixes**
* Safer pagination with per-page JSON handling, limits and improved
termination diagnostics; ensures pageNum is used when needed.

* **Tests**
* New comprehensive pagination tests and updates to existing tests to
reflect multi-page behavior.

* **Chores**
* Added a utility to safely coerce page numbers; minor utility signature
cleanup.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/kleinanzeigen_bot/utils/misc.py

@@ -16,12 +16,55 @@ from . import i18n
 T = TypeVar("T")
 
 
+def coerce_page_number(value:Any) -> int | None:
+    """Safely coerce a value to int or return None if conversion fails.
+
+    Whole-number floats are accepted; non-integer floats are rejected.
+
+    Args:
+        value: Value to coerce to int (can be int, str, float, or any type)
+
+    Returns:
+        int if value can be safely coerced, None otherwise
+
+    Examples:
+        >>> coerce_page_number(1)
+        1
+        >>> coerce_page_number("2")
+        2
+        >>> coerce_page_number(3.0)
+        3
+        >>> coerce_page_number(3.5) is None
+        True
+        >>> coerce_page_number(True) is None  # Not 1!
+        True
+        >>> coerce_page_number(None) is None
+        True
+        >>> coerce_page_number("invalid") is None
+        True
+        >>> coerce_page_number([1, 2, 3]) is None
+        True
+    """
+    if value is None:
+        return None
+    if isinstance(value, bool):
+        return None
+    if isinstance(value, float):
+        if value.is_integer():
+            return int(value)
+        return None
+    try:
+        return int(value)
+    except (TypeError, ValueError):
+        return None
+
+
 def ensure(
-        condition:Any | bool | Callable[[], bool],  # noqa: FBT001 Boolean-typed positional argument in function definition
-        error_message:str,
-        timeout:float = 5,
-        poll_frequency:float = 0.5
-    ) -> None:
+    condition:Any | bool | Callable[[], bool],  # noqa: FBT001 Boolean-typed positional argument in function definition
+    error_message:str,
+    timeout:float = 5,
+    poll_frequency:float = 0.5,
+) -> None:
     """
     Ensure a condition is true, retrying until timeout.
 
@@ -152,12 +195,7 @@ def parse_decimal(number:float | int | str) -> decimal.Decimal:
             raise decimal.DecimalException(f"Invalid number format: {number}") from ex
 
 
-def parse_datetime(
-    date:datetime | str | None,
-    *,
-    add_timezone_if_missing:bool = True,
-    use_local_timezone:bool = True
-) -> datetime | None:
+def parse_datetime(date:datetime | str | None, *, add_timezone_if_missing:bool = True, use_local_timezone:bool = True) -> datetime | None:
     """
     Parses a datetime object or ISO-formatted string.
 
@@ -184,10 +222,7 @@ def parse_datetime(
     dt = date if isinstance(date, datetime) else datetime.fromisoformat(date)
 
     if dt.tzinfo is None and add_timezone_if_missing:
-        dt = (
-            dt.astimezone() if use_local_timezone
-            else dt.replace(tzinfo = timezone.utc)
-        )
+        dt = dt.astimezone() if use_local_timezone else dt.replace(tzinfo = timezone.utc)
 
     return dt