fix(workflows): Correctly convert boolean filter values for Snuba queries

[sentry]

This PR addresses a QueryExecutionError originating from ClickHouse when evaluating workflow alert conditions that include filters like error.handled = false.

Root Cause:
The convert_filter_to_snuba_condition function in sentry/workflow_engine/handlers/condition/event_frequency_query_handlers.py was passing the string literal 'false' directly to Snuba for the error.handled attribute. This attribute maps to the exception_stacks.mechanism_handled column in ClickHouse, which is of type UInt8. ClickHouse cannot implicitly convert the string 'false' to a UInt8, leading to a type conversion error.

Additionally, a subtle bug was identified in the handling of error.unhandled. The expression not condition["value"] (where condition["value"] could be the string 'false') would incorrectly evaluate to False because any non-empty string is truthy in Python. This meant that a filter for error.unhandled = false was being translated to handled = False instead of the semantically correct handled = True.

Fix:
The logic for both error.handled and error.unhandled within convert_filter_to_snuba_condition has been updated and consolidated. It now correctly parses the string value ('true' or 'false') from the condition into a Python boolean (True or False) before constructing the Snuba query. This ensures that ClickHouse receives a compatible boolean/integer type for the UInt8 column, resolving the type conversion error and correctly interpreting the error.unhandled filter.

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Fixes SENTRY-5PE8