Extract DomainAccessManager static helpers (getAccessValues, getAllValue, etc.) to a public utility class

Problem

Drupal\domain_access\DomainAccessManager exposes 5 public static methods that ecosystem modules consume directly:

• getAccessValues(FieldableEntityInterface $entity, $field_name = ...)
• getAllValue(FieldableEntityInterface $entity)
• getDefaultValue(FieldableEntityInterface $entity, FieldDefinitionInterface $definition)
• clearStaticCache($entity_id = NULL, $entity_type_id = NULL)
• userCanAccessDomain(AccountInterface $account, $domain_id): UserDomainAccess

Two of these (getAccessValues() and getAllValue()) are de-facto public API in the ecosystem:

• domain_path — 11 call sites across 3 files (DomainPathHelper, DomainPathFormHelper, DomainPathItem)
• domain_shorturl — 1 call site in DomainShortUrlFormHooks
• domain_extras/domain_access_linkit — 1 call site in AssignedDomainsNodeMatcher

The (now-reverted) @internal tag on DomainAccessManager in #3584261 surfaced the conflict: marking the service @internal while keeping its statics public sends mixed signals and forces every ecosystem call site to add @phpstan-ignore method.internalClass.

Proposal

Extract the static helpers into a dedicated public utility class Drupal\domain_access\Utility\DomainAccessFields (name TBD). Same shape — pure entity-field readers, no DI needed, mirrors how #3583423 extracted DomainStorage::createMachineName() to Drupal\domain\Utility\MachineName::fromString().

On DomainAccessManager, the existing statics become @deprecated in domain:3.x.0 and is removed from domain:4.0.0 BC wrappers that delegate to the new class. Sibling modules migrate via a clean use Drupal\domain_access\Utility\DomainAccessFields; swap.

Once 4.0.0 lands, the wrappers + the cross-module phpstan friction disappear and DomainAccessManager can go back to being @internal cleanly.

Rationale

• Mirrors the precedent set by MachineName::fromString() in #3583423 — separate "service" (instance, internal-able) from "field utility" (public).
• The methods are pure-data readers with no instance state — having them on a service class with a __construct() that only exists to host static methods is awkward.
• Sibling modules get a clean migration path without needing DI churn for what should be a simple field-value lookup.
• Lets the next deprecation pass (4.0.0) restore @internal on DomainAccessManager without breaking ecosystem callers.

Out of scope

• The 3 less-used statics (getDefaultValue, clearStaticCache, userCanAccessDomain) — confirm whether to migrate them too. getDefaultValue is referenced by field defaults so its callable signature is constrained; userCanAccessDomain looks fine to migrate; clearStaticCache is internal-only in practice.
• Other modules' equivalent helpers (none found in the survey — DomainAccessPermissions, DomainAccessHelper, DomainSourceHelper, DomainElementManager have no public statics worth migrating).

Target

Deprecate in domain:3.x, remove wrappers in domain:4.0.0.

Discovered while auditing #3584261.