Make raise_on_notset default to True with environment variable override

[flipdazed]

This PR changes how the rule engine handles missing fields by making it stricter by default while maintaining flexibility for users who need different behaviour.

Key changes:

1. Changed _raise_on_notset to default to True instead of False
2. Added environment variable RULE_ENGINE_RAISE_ON_NOTSET to globally control this behavior
   • Set to "true" or "1" to enable raising (default)
   • Set to any other value to disable raising
3. Maintained the ability to override per-rule using __raise_on_notset parameter
4. Updated tests to:
   • Explicitly set __raise_on_notset=False where needed
   • Add comprehensive test coverage for environment variable behavior
   • Verify interaction between env var and per-rule settings

This change makes the behaviour more explicit and safer by default, as it will now alert users when they try to evaluate rules against missing fields rather than silently failing. Users who need the old behavior can either:

• Set RULE_ENGINE_RAISE_ON_NOTSET=false globally
• Set __raise_on_notset=False on specific rules

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (ade82bd) to head (01946be).

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Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines          353       371   +18     
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+ Hits           353       371   +18     

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