chore: Raise on primary key dtype mimsatch

[Marius Merkle (MariusMerkleQC)]

Motivation

Solves #52.

Changes

Instead of silently ignoring an invalid primary key (e.g. due to data type mismatch), I suggest raising a PrimaryKeyError as we do for other instances where the passed primary_key is in fact not a valid primary key.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3bbdce9) to head (aa4131a).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #56   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         1140      1138    -2     
=========================================
- Hits          1140      1138    -2     

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[Copilot]

🟢 Approval recommended

Pull request overview

This PR changes Diffly’s primary-key validation behavior to fail fast when the left/right primary key column dtypes are incompatible, instead of warning and silently disabling the primary key. This makes dtype mismatches explicit and prevents comparisons from continuing in an unintended “no primary key” mode.

Changes:

• Replace the previous UserWarning + primary_key = None fallback with raising PrimaryKeyError when join-key dtypes are incompatible.
• Update the corresponding unit test to assert the new exception-based behavior.

File summaries

File	Description
diffly/comparison.py	Converts primary key dtype incompatibility handling from a warning-and-fallback to raising PrimaryKeyError.
tests/test_dataframe_comparison.py	Updates the dtype-mismatch test to expect PrimaryKeyError instead of a warning.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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