Improve snapshot mismatch reporting

[AKuederle]

Summary

• wrap snapshot comparison failures in a snapshot-specific assertion with cleaner pytest output
• include snapshot file paths and detailed pandas/numpy mismatch details in failure messages
• add integration tests covering dataframe and string mismatch reporting

Testing

• .venv/bin/pytest tests/test_misc/test_snapshot_testsuite.py tests/test_misc/test_snapshot_failure_reporting.py -q
• .venv/bin/ruff check src/tpcp/testing/_regression_utils.py tests/test_misc/test_snapshot_failure_reporting.py

[codecov-commenter]

Codecov Report

❌ Patch coverage is 18.60465% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.87%. Comparing base (2574782) to head (5d328eb).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tpcp/testing/_regression_utils.py	18.60%	35 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
- Coverage   78.67%   77.87%   -0.80%     
==========================================
  Files          29       29              
  Lines        2513     2545      +32     
==========================================
+ Hits         1977     1982       +5     
- Misses        536      563      +27     

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