Write tests for the DomainPathAlias validation constraints

Test coverage for all validation constraints

Added Kernel tests for all four validation constraints in the domain_path module. The tests are organized with a shared base class to avoid duplication:

• DomainPathKernelTestBase — shared setUp (entity schemas, config, 2 test domains, uid 1, path alias storage), generic filterViolations() helper, and createPathAlias() helper
• DomainPathUniqueAliasTest — 6 tests for DomainPathUniquePathAliasConstraint (path_alias entity-level)
• DomainPathFieldConstraintsTest — 7 tests for the three DomainPathItem field-level constraints

DomainPathUniqueAliasTest (path_alias uniqueness scoped by domain)

• testSameAliasOnDifferentDomainsIsAllowed — /about on domain A + /about on domain B -> no violation
• testSameAliasOnSameDomainConflicts — /about on domain A twice -> violation ("already in use in this domain")
• testGlobalAliasDoesNotConflictWithDomainAlias — /about on domain A + global /about -> no violation
• testDomainAliasDoesNotConflictWithGlobalAlias — global /about + /about on domain A -> no violation
• testDuplicateGlobalAliasesConflict — global /about twice -> violation ("already in use")
• testEditingExistingAliasDoesNotSelfConflict — re-validating an existing alias -> no violation

DomainPathFieldConstraintsTest (DomainPathItem field-level)

DomainPathSlash:

• testSlashConstraintRejectsAliasWithoutSlash — alias without leading / -> violation
• testSlashConstraintAllowsAliasWithSlash — alias with leading / -> no violation
• testSlashConstraintAllowsEmptyAlias — empty alias -> no violation

DomainPathUnique:

• testUniqueConstraintRejectsDuplicateAlias — alias matching existing domain path alias -> violation
• testUniqueConstraintAllowsNewAlias — novel alias -> no violation

DomainPathUserAccess:

• testUserAccessConstraintRejectsUnauthorizedUser — user without domain access -> violation
• testUserAccessConstraintAllowsAuthorizedUser — admin with access -> no violation

The field constraints tests validate through full entity validation on node entities with the computed domain_path field, using domain_access module for realistic user access scenarios.