Fix E3031 false positive for boolean values with pattern

[kddejong]

Summary

Fix incorrect E3031 error reported for boolean properties that have a pattern keyword in the schema (e.g., SES ConfigurationSet's ReputationMetricsEnabled and SendingEnabled).

Root Cause

The pattern function in _keywords.py used str(instance) to convert non-string values before regex matching. Python's str(True) produces "True", but CloudFormation stringifies booleans as lowercase "true". The upstream SES ConfigurationSet schema defines these properties as type: boolean with pattern: "true|false", so "True" didn't match.

Fix

Changed str(instance) to json.dumps(instance) which produces "true"/"false" for booleans — matching CloudFormation's actual behavior.

Testing

• Verified against real CloudFormation deployments that boolean true is the correct type
• Added unit test for boolean pattern matching
• Updated existing test to reflect correct stringification
• All 354 existing tests pass

Fixes #4484

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.47%. Comparing base (79871e2) to head (37f3003).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4485   +/-   ##
=======================================
  Coverage   93.47%   93.47%           
=======================================
  Files         459      459           
  Lines       15022    15023    +1     
  Branches     2906     2906           
=======================================
+ Hits        14042    14043    +1     
  Misses        601      601           
  Partials      379      379           

Flag	Coverage Δ
unittests	93.47% <100.00%> (+<0.01%)	⬆️

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