Add 'detect' task type: return bbox without running OCR

[surajama]

Adds a new detect task type to label_task_mapping. When a region label (e.g. table) is mapped to detect, the local pipeline skips the OCR API call and returns the region with its bbox_2d intact and content: "". This lets callers crop table regions by bounding box without paying the token cost of table OCR.

Changes:

• pipeline/pipeline.py: route detect regions through pending_skip (no OCR submitted), set content="" on drain
• postprocess/result_formatter.py: exempt detect regions from the empty-content filter so they survive into json_result
• config.py: add id2label as an explicit LayoutConfig field (was previously only accessible via model_extra, causing AttributeError when the field was absent from a partial config)
• examples/detect/: self-hosted example config + run script

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