fix(functiongemma): parser and template bug fixes
Parser: Use state machine to find closing brace, properly handling `}`
characters inside `<escape>` delimiters. Previously used [^}]* regex
which would break on values like `{key:<escape>foo}bar<escape>}`.
Template: Output tool calls in native FunctionGemma format
`{key:<escape>value<escape>}` instead of JSON format `{"key":"value"}`.
This ensures the model sees correct examples in conversation history.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
