Use PSR-17 createRequest() instead of deprecated getMessageFactory()

[mwansinck]

Problem

bpost::executeQuery() is the only place that still calls the deprecated AbstractHttpProvider::getMessageFactory(). That method returns an Http\Message\MessageFactory, an interface that exists solely in the archived/abandoned php-http/message-factory package. As a result the provider hard-requires that abandoned package at runtime, and fails with:

Interface "Http\Message\MessageFactory" not found

whenever it isn't present (e.g. when consumers prune abandoned dependencies). See #16.

Fix

Switch the request construction to AbstractHttpProvider::createRequest() (from geocoder-php/common-http), which already builds the request through PSR-17 factories and has an identical argument signature ($method, $uri, array $headers = [], ?string $body = null). This is exactly the approach every other geocoder-php provider uses.

-        $request = $this->getMessageFactory()->createRequest(
+        $request = $this->createRequest(
             'POST',
             $url,
             [
                 'Content-Type' => 'application/json',
             ],
             $data
         );

This removes the last reference to Http\Message\MessageFactory, so the provider no longer pulls in php-http/message-factory. No behavioural change — the produced request is identical.

Testing

Verified against a real project: with this change applied and php-http/message-factory removed, the bpost geocoding integration tests (mocked bpost responses) still pass. Without the change they fatal on the missing interface.

Fixes #16