The stock ``base_geocoder._call_openstreetmap`` sends a generic Odoo
``User-Agent`` shared across thousands of instances, which Nominatim
rate-limits and blocks (HTTP 429 "Too many requests"), so mass
geolocation fails regardless of client-side throttling.
Override ``_call_openstreetmap`` to:
- send a deployment-specific ``User-Agent`` read from a new system
parameter ``partner_geolocalize_usability.user_agent`` (shipped with an
obvious placeholder so an unconfigured instance fails loudly instead of
silently reusing the banned generic agent);
- throttle requests to one per second (``time.sleep``) before each call,
complying with the Nominatim usage policy even when callers loop over
many partners;
- return ``None`` instead of raising ``IndexError`` when Nominatim
returns an empty result for an unknown address.
Document the required system parameter in the README and add tests
covering the User-Agent selection, throttling and empty/blank results.
