Resolve every constant entity type ID in getStorage() return type

[mglaman]

Split out of #1027. Type inference improvement, no configuration change.

What changed

getStorage() now resolves every constant string in its argument, so getStorage($cond ? 'node' : 'user') infers NodeStorage|UserStorage instead of silently using the first string. Chained calls on that union return every branch: ->load(1) is Node|User|null, ->loadMultiple() is array<int, Node|User>, and ->create([]) is Node|User. Narrowing with instanceof on a member still works.

Three pieces make the union sound:

• EntityStorageType compares by entity type ID. isSuperTypeOf(), equals(), and the cache-level description treat two storages with different entity type IDs as distinct even when they share a class. Without this, NodeStorage|SqlContentEntityStorage collapsed to the base class and the node branch was lost. User-facing descriptions are unchanged.
• EntityStorageDynamicReturnTypeExtension resolves each union member on its own instead of asking resolveFromStorage() for the first registered match.
• An unknown entity type ID falls back to the declared return type. getStorage($cond ? 'node' : 'not_an_entity_type') is EntityStorageInterface. Before, an unknown ID produced an interface type tagged with the ID, which served no consumer.

The Concat/MethodCall AST sniffing is gone; Scope::getType() already folds constant expressions. The extension declares ?Type and returns null to fall back, and no longer computes the declared return type eagerly on every call.

Known display quirk

Two config entity types that both use ConfigEntityStorage describe as ConfigEntityStorage#1|ConfigEntityStorage#2. The #N suffix is PHPStan's own rendering for union members whose names collide. They are different storages, and the chained result (Block|ConfigEntityUsingDefaultStorage) reads normally.

Testing

Fixture cases cover: the union on getStorage() itself, chained load(), loadMultiple(), and create() in both operand orders, two content storages sharing SqlContentEntityStorage, two config storages sharing ConfigEntityStorage, a content and config storage together (array<int|string, Block|Node>), a known plus unknown ID, a fully dynamic ID, a first-class callable, and instanceof narrowing. Full suite, self-analysis, and phpcs are green.

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