fix(dotprompt-dart,dotprompt-java,dotprompt-rs): support dotted partial names

[anthonyAttentive]

Support for dots in partial names (e.g., {{> greeting.in_french}}) is currently inconsistent across SDKs:

SDK	Status
Python	Supported — regex allows . via [a-zA-Z0-9_.-]+
JS/Go	Supported — AST-based parsing naturally allows dots
Dart	Not supported
Java	Not supported
Rust	Not supported

This PR adds . to the partial name regex character class in Dart, Java, and Rust, and adds a shared partial_name_with_full_regex spec test case to spec/partials.yaml to validate partial name regex consistency across all SDKs.

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