Add enqueue object and lock structure for PDF documents

[Copilot]

Adds ABAP locking infrastructure for PDF resource management.

Changes

• src/zpdf_lock.tabl.xml: Lock structure defining key fields (MANDT, PDF_ID)
• src/ezpdf.enqu.xml: Enqueue object EZPDF with exclusive lock mode on both fields

Usage

Standard ABAP enqueue/dequeue pattern for preventing concurrent modification:

" Lock PDF document
CALL FUNCTION 'ENQUEUE_EZPDF'
  EXPORTING
    mandt  = sy-mandt
    pdf_id = lv_document_id.

" ... modify PDF resource ...

" Unlock PDF document  
CALL FUNCTION 'DEQUEUE_EZPDF'
  EXPORTING
    mandt  = sy-mandt
    pdf_id = lv_document_id.

Original prompt

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This section details on the original issue you should resolve

<issue_title>create a new enqueue object and corresponding structure</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes create a new enqueue object and corresponding structure #7

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