Parse nested unions inside generic types

[sirreal]

What changed

• split PHPDoc unions only at top-level bracket depth
• preserve nested generic syntax while resolving aliases recursively
• add export coverage for the exact nested union from issue Nested union types inside generics are split incorrectly #268

Why

The pinned ReflectionDocBlock collection splits every pipe character before it understands generic nesting. That corrupts list<string|MessagePart|array> into separate top-level types and applies namespace expansion to the fragments.

Testing

• focused Export_Docblocks regression: 1 test, 2 assertions
• npm test: 23 tests, 127 assertions

Fixes #268.

Theme rendering remains tracked separately in WordPress/wporg-developer#573.