Origin/feature/port onehot operator

[junseokShim]

Changes to align with TFLM conventions / review feedback

• Flattened the namespace usage to namespace tflite { ... } and kept file-local helpers in an anonymous namespace
• Renamed the output shape check helper to reflect its actual behavior (validation, not resizing)
• Updated comment wording to refer to Memory Planner (not “Offline Memory Planner”)
• Updated the micro test to use tflite::Register_ONE_HOT() via the standard registration header (no forward declaration)

bug=fixes #3078

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[junseokShim]

@ddavis-2015
Hi! I’m an external contributor so I can’t apply CI labels.
one_hot_test passes locally and CLA is resolved.
Could you please add ci:run (or ci:run_full if needed) and cla: yes so CI can run? Thanks!****