[FIX] partner_geolocalize_usability: avoid Nominatim HTTP 429 rejections

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.

partner_geolocalize_usability/README.rst

@@ -21,7 +21,25 @@ It also modify the partner form vieww for a better understanding of the geolocat
 Configuration
 =============
 
-No configuration needed.
+The OpenStreetMap Nominatim geocoding service requires a genuine, identifying
+``User-Agent`` header with a valid contact address (see the `Nominatim usage
+policy <https://operations.osmfoundation.org/policies/nominatim/>`_). The
+default Odoo ``User-Agent`` is shared by many instances and gets rate-limited or
+blocked (HTTP 429 "Too many requests").
+
+To fix this, set the system parameter (Settings > Technical > System
+Parameters):
+
+- **Key**: ``partner_geolocalize_usability.user_agent``
+- **Value**: a User-Agent identifying your deployment and a valid contact,
+  e.g. ``acme-crm/1.0 (contact: gis@acme.example)``
+
+The module ships this parameter with a placeholder value; you MUST replace it
+with your own, otherwise Nominatim will reject the requests.
+
+Requests to Nominatim are additionally throttled to one request per second to
+comply with the usage policy, so mass geolocation of many partners is slow by
+design (roughly one partner per second).
 
 Known issues / Roadmap
 ======================