[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/data/ir_config_parameter.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<odoo>
+  <!--
+    User-Agent sent to the OpenStreetMap Nominatim geocoding service.
+
+    Nominatim's usage policy requires a genuine User-Agent identifying the
+    application and a valid contact address. The value below is a PLACEHOLDER:
+    each deployment MUST replace it with its own service name and contact,
+    e.g. "acme-crm/1.0 (contact: gis@acme.example)". Leaving the placeholder
+    will result in HTTP 429 (Too many requests) from Nominatim.
+  -->
+  <record id="param_nominatim_user_agent" model="ir.config_parameter">
+    <field name="key">partner_geolocalize_usability.user_agent</field>
+    <field name="value">SET-ME partner_geolocalize_usability (contact: you@example.invalid)</field>
+  </record>
+</odoo>