Draft: Issue #3600697: Handle array namespace paths in attribute route discovery

Problem

AttributeRouteDiscovery assumed each container.namespaces value was a single string path and appended /Controller directly to it. Core also supports namespace values as arrays of PSR-4 base directories, so an array-valued namespace produced an Array to string conversion warning during route discovery.

Solution

This updates attribute route discovery to normalise namespace values to a list of directories before scanning controller paths. It keeps existing string-path behaviour and supports multiple namespace directories.

A regression test covers an array-valued namespace with multiple paths, including a missing path before the valid controller directory.

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Superseded by !16058