fix(jsonapi): keep flat custom params with flat page (#8216)

[soyuka]

Summary

Fixes #8216 — regression introduced in #8193.

JsonApiProvider was setting _api_filters from flat pagination params
(page, itemsPerPage, pagination, partial) even when no filter[...]
bracket was present. ReadProvider:63-65 short-circuits its raw query
fallback (RequestParser::parseRequestParams) whenever _api_filters is
non-null, so any flat custom filter (?city_id=3&order[distance]=asc)
combined with page=N was silently dropped.

Fix

Parse the raw query string in JsonApiProvider and merge it as a base
layer under the existing JSON:API transformations:

$queryString = RequestParser::getQueryString($request);
$rawParams = $queryString ? RequestParser::parseRequestParams($queryString) : [];
$filters = array_replace($rawParams, $filters);

JSON:API transformations (sort → order, filter[foo] → foo, flat
pagination keys) still take precedence via array_replace.

Tests

• Unit (JsonApiProviderTest::testProvidePreservesFlatCustomQueryParamsWithoutBracketFilter)
  asserts _api_filters contains city_id, order, and page after the
  provider runs.
• Functional (JsonApiFlatPaginationTest::testFlatCustomParamWithFlatPagePreservesFilter)
  hits /dummies?name=foo&page=1 w/ Accept: application/vnd.api+json and
  asserts the SearchFilter is applied (totalItems === 5 out of 6 fixtures).
  Verified the functional test fails on the prior code (returns 6).

Test plan

• vendor/bin/phpunit src/JsonApi/Tests/State/JsonApiProviderTest.php
• vendor/bin/phpunit tests/Functional/JsonApiFlatPaginationTest.php
• Confirmed functional test fails without the patch (asserts 6 === 5).