Fix logical NOT for string-backed boolean values

[fallenmi]

Fixes #1122.

Literal values passed to a SubTree remain strings in its blackboard. Script unary logical NOT rejected every string, including values that use the standard boolean spellings accepted by convertFromString<bool>.

Allow ! to interpret only true/True/TRUE/1 and false/False/FALSE/0. Other strings still raise the existing Invalid operator for std::string error, and the blackboard values remain unchanged for string consumers.

Regression coverage includes:

• the reported _skipIf="!enabled" path;
• lowercase, capitalized, uppercase, and remapped 1/0 values;
• preservation of the original string values;
• rejection of a numeric-prefix string that is not a boolean.

Validation

• cmake --build build --target behaviortree_cpp_test --parallel 8
• ./build/tests/behaviortree_cpp_test --gtest_filter='SubTree.*:Preconditions.*:ParserTest.*:PortTest.*:PortTypeRules.*' — 110 passed
• uvx pre-commit run -a
• git diff --check

AI assistance disclosure

OpenAI Codex reproduced the issue, drafted the code and tests, checked the visible upstream overlap surface, and ran the validation listed above. The account owner remains responsible for the contribution.