Encode boolean query parameters as lowercase (#3516)

Closes #3304.

**Problem:** `add_parameters_to_url` passes Python booleans straight to
`urllib.parse.urlencode`, which renders them capitalized. So
`get_notifications(all=True)` sends `all=True` instead of `all=true`.
GitHub expects lowercase booleans and ignores the capitalized value, so
the parameter has no effect.

```python
>>> import urllib.parse
>>> urllib.parse.urlencode([("all", True)])
'all=True'
```

**Fix:** Convert boolean values to lowercase strings before encoding.

**Test:** Added a case to `testAddParametersToUrl` covering
`all`/`participating`. It fails on the current code
(`all=True&participating=False`) and passes with the fix.

---------

Co-authored-by: Noethix55555 <277300782+Noethix55555@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Enrico Minack <github@enrico.minack.dev>