chore: Add dedicated PrimaryKeyError

[MariusMerkleQC]

Motivation

Solves #16

Changes

• Introduce a custom PrimaryKeyError exception in a new _exceptions.py file
• Make the PrimaryKeyError easily importable: from diffly import PrimaryKeyError
• Replace existing primary key errors "masked" as ValueError with PrimaryKeyError

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fb90a0e) to head (ee3a10c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9        10    +1     
  Lines          704       707    +3     
=========================================
+ Hits           704       707    +3     

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[Copilot]

Pull request overview

Adds a dedicated PrimaryKeyError exception to make primary-key-related failures in diffly easier to catch distinctly from generic ValueErrors (per issue #16).

Changes:

• Introduces diffly.PrimaryKeyError (defined in a new internal module and re-exported at package level).
• Updates DataFrameComparison/compare_frames primary key validation and “missing primary key” join checks to raise PrimaryKeyError.
• Adjusts affected tests to expect PrimaryKeyError instead of ValueError.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
diffly/_exceptions.py	Adds the new PrimaryKeyError exception type.
diffly/__init__.py	Re-exports PrimaryKeyError to support from diffly import PrimaryKeyError.
diffly/comparison.py	Replaces primary-key-related ValueError raises with PrimaryKeyError.
tests/test_dataframe_comparison.py	Updates primary key validation tests to assert PrimaryKeyError.
tests/test_joined.py	Updates missing-primary-key join test to assert PrimaryKeyError.
tests/test_fraction_same.py	Updates missing-primary-key fraction_same test to assert PrimaryKeyError.

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[EgeKaraismailogluQC]

Looks good to me!