Add unit tests for AddCommand and HelpCommand

[dmccoystephenson]

Summary

• Unit tests are added for AddCommand and HelpCommand, the only two command classes previously left without test coverage (EditCommand, RemoveCommand, and DefaultCommand already had tests).
• Existing Mockito-based conventions from EditCommandTest/RemoveCommandTest are followed: item/inventory/meta are mocked, lore mutation is verified via setLore, and rejection paths are asserted to return false.
• A latent bug is characterized rather than fixed, per this cycle's test-expansion scope: AddCommand.execute(CommandSender, String[]) has no args.length == 0 guard (unlike its EditCommand/RemoveCommand siblings), so an uncaught IllegalArgumentException is thrown instead of a graceful usage message when /pl add is invoked with no arguments. This is tracked separately in AddCommand throws uncaught exception instead of usage message when args are missing #10, filed alongside this PR, and left for a future implementation cycle rather than being fixed here.

Test plan

• mvn test — all 19 tests pass (12 pre-existing + 7 new, across AddCommandTest and HelpCommandTest)
• No production code was modified; only new test files were added

No tracking issue is closed by this PR — the cycle's backlog consisted of no open issues, so this cycle's work was selected from Phase 2's Stage B (unit-test expansion) rather than issue implementation. #10 was filed as a followup and is left open for a future cycle.

This PR description was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — the diff touches only two new test files (AddCommandTest.java, HelpCommandTest.java); no production code, no unrelated formatting/renames.
• Tests-new: PASS — every public method on AddCommand and HelpCommand (execute(CommandSender) and execute(CommandSender, String[])) is exercised by at least one test.
• Tests-fix (empirical): N/A — this cycle is Stage B (test-expansion), not a bug-fix cycle. The one apparent bug found (AddCommand throwing IllegalArgumentException on empty args) is intentionally left unfixed per the characterization-only scope, filed as AddCommand throws uncaught exception instead of usage message when args are missing #10, and its test asserts the current throwing behavior rather than a fix.
• Sibling structure: PASS — both new test files mirror the @BeforeEach mock-setup and per-branch @Test structure of EditCommandTest.java/RemoveCommandTest.java (same mock objects, same verify/assertTrue/assertFalse idioms).
• Sibling renames: N/A — no identifiers were renamed.
• Docs: PASS — no behavior changed, so HelpCommand.java, COMMANDS.md, USER_GUIDE.md, CONFIG.md, and CHANGELOG.md all still accurately describe the plugin; none required an update for this PR.
• Issue resolution: N/A — no Closes #N in this PR. The cycle had an empty issue backlog, so work was pulled from Phase 2 Stage B instead. AddCommand throws uncaught exception instead of usage message when args are missing #10 was filed as a byproduct, not closed by this PR.
• CI: PASS — build check green on the PR head SHA (mvn test, 19/19 passing).

Repo-specific rubric:

• Command trio parity: N/A — no validation/permission logic was changed in AddCommand/EditCommand/RemoveCommand; the parity gap discovered (AddCommand missing the args.length == 0 guard present in its two siblings) is documented as a characterization test and tracked in AddCommand throws uncaught exception instead of usage message when args are missing #10 rather than fixed here.

Summary: two new test files add coverage for the previously-untested AddCommand and HelpCommand; a latent AddCommand bug was found and characterized (not silently patched) with a companion issue (#10) for a future implementation cycle.

This comment was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).