refactor: unify MockUserLookup into shared test helper package. Closes #439.

[fdcastel]

Consolidates the duplicated MockUserLookup struct into a single, unified implementation in a new test/testutil package. The unified struct supports both single-user and multi-user lookup scenarios, as requested in the issue.

Closes #439

Changes

• New file: test/testutil/mockuserlookup.go

  • Unified MockUserLookup struct implementing policy.UserLookup
  • Supports single-user lookups via User field (existing behavior)
  • Supports multi-user lookups via Users map[string]*user.User field (new capability)
  • When both are set, Users map is checked first, then User as fallback
  • Shared ValidUser test fixture (was also duplicated)
  • Compile-time interface check (var _ policy.UserLookup = &MockUserLookup{})

• Updated: policy/policyloader_test.go

  • Removed duplicate MockUserLookup struct, Lookup() method, and interface check
  • Added local type alias (type MockUserLookup = testutil.MockUserLookup) to keep existing test code concise
  • ValidUser now references testutil.ValidUser

• Updated: commands/add_test.go

  • Removed duplicate MockUserLookup struct and Lookup() method (which had the comment "Duplicates code from multipolicyloader_test.go")
  • Added local type alias and ValidUser reference, same as above

Design decisions

• Type aliases (type MockUserLookup = testutil.MockUserLookup) are used in each test file so that existing test code doesn't need to be prefixed with testutil. everywhere. This keeps the diff minimal and the test code readable.
• User field preserved for backward compatibility — all existing usages like &MockUserLookup{User: ValidUser} continue to work with zero changes.
• Users map field added to support multi-user test scenarios as mentioned in the issue.

[Copilot]

Pull request overview

This PR consolidates duplicated MockUserLookup test mocks into a single shared helper (test/testutil) to reduce repetition and support both single-user and multi-user lookup scenarios, addressing #439.

Changes:

• Added test/testutil.MockUserLookup implementing policy.UserLookup, supporting both User and Users (map) modes plus a shared ValidUser fixture.
• Updated policy/policyloader_test.go to use the shared helper via a local type alias and testutil.ValidUser.
• Updated commands/add_test.go similarly to use the shared helper and remove the duplicated mock.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/testutil/mockuserlookup.go	Introduces unified MockUserLookup and shared ValidUser fixture for tests.
policy/policyloader_test.go	Removes local mock implementation; aliases and reuses testutil helper.
commands/add_test.go	Removes local mock implementation; aliases and reuses testutil helper.

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