Add mdPatternDS() - Server-side missing data pattern with disclosure control

[ESCRI11]

Implements server-side aggregate function for analyzing missing data patterns using mice::md.pattern().

Features:

• Generates missing data pattern matrix
• Suppresses patterns with counts below nfilter.tab threshold
• Returns pattern matrix with validity status

Server-side component for ds.mdPattern() client function.

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Codecov Report

❌ Patch coverage is 0% with 37 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v6.3.5-dev@ea12b73). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
R/mdPatternDS.R	0.00%	37 Missing ⚠️
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Additional details and impacted files

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  Hits                  ?     1686           
  Misses                ?     4080           
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