test: migrate remaining trivial behat features to ApiTestCase

Replaces 13 behat features (xml, issues, sub_resources, push_relations,
mercure, json, http_cache, filter) with functional ApiTestCase tests
under tests/Functional/. Scenarios map 1:1; sequential behat scenarios
(http_cache tags, m2m tag_collector) are consolidated into single test
methods that walk through the same sequence with NullPurger resets.

New PropertyFilterTest covers resource-relation sparse fieldsets through
both #[ApiFilter(PropertyFilter::class)] and #[QueryParameter(filter: ...)]
on a new isolated fixture pair (SparseFieldsetParent/Child); the non-
resource scenario is already covered by NonResourceTest.

TagCollectorTest installs the custom TagCollectorCustom via
\$container->set() + KernelBrowser::disableReboot() so the override
survives across requests — same pattern as Behat HttpCacheContext.

Doctrine, GraphQL, MongoDB, Main, Elasticsearch, Security and Serializer
feature sets are intentionally out of scope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: soyuka