test: cover REST operations edge cases

[erseco]

Summary

• add integration coverage for missing assignment/removal parameters returning 400
• verify non-task posts are rejected with 404 and remain untouched
• cover normalization of scalar legacy assigned_users metadata
• verify duplicate assignments are idempotent, emit no duplicate hook, and do not rotate the edit generation
• verify no-op removals preserve assignment state and the active generation
• verify real removals preserve remaining assignees and invalidate open editing sessions
• verify due-date requests without a value are no-ops
• verify due-date values are sanitized, persisted, and invalidate stale sessions

Why

The REST operations controller was recently extracted from Decker_Tasks. Existing tests cover route registration and the main mutation flows, but not the error responses, legacy metadata shapes, and idempotent/no-op behavior. These cases are important because unnecessary generation rotation interrupts active editors, while missed rotation can allow stale forms to overwrite out-of-band changes.

Test plan

CI should run the complete PHPUnit, lint, Plugin Check, PHPMD, JavaScript, and E2E suites. This PR only adds integration tests and does not change production behavior.

[github-actions]

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