Add mock data and testing for multi tables' related imp

[Wh1isper]

Description

Patch for #96 remining work...

Motivation and Context

• Adding mock data for multiple tables
• Adding unit tests for MetadataCombiner.

MetadataCombiner basic availability now.

How has this been tested?

Types of changes

• Maintenance (no change in code, maintain the project's CI, docs, etc.)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[sweep-ai]

Apply Sweep Rules to your PR?

• Apply: All new business logic should have corresponding unit tests.
• Apply: Refactor large functions to be more modular.
• Apply: Add docstrings to all functions and file headers.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (160f6a2) 76.63% compared to head (eae5df9) 78.68%.

Files	Patch %	Lines
sdgx/manager.py	77.77%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   76.63%   78.68%   +2.05%     
==========================================
  Files          59       59              
  Lines        2435     2463      +28     
==========================================
+ Hits         1866     1938      +72     
+ Misses        569      525      -44     

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