Add PyBuilder to root aggregate and cover non-macro gaps with unit tests

[Yicong-Huang]

Task Summary

The PyBuilder sbt project is defined in the root build.sbt but not listed in the TexeraProject aggregate(...). As a result, running sbt test or sbt compile from the repository root silently skips it. CI explicitly invokes sbt "PyBuilder/jacoco" so coverage is still reported, but the inconsistency means a local sbt test does not match the CI matrix.

Additionally, while PythonLexerUtils and the pyb"..." macro are well-covered, a few non-macro pieces of PythonTemplateBuilder are not exercised by any spec:

• PythonTemplateBuilder.+(String) which throws UnsupportedOperationException
• fromInterpolated parts/args length precondition
• render() CR/CRLF → LF normalization
• PyStringTypes.{EncodableStringFactory, PyLiteralFactory} factories and their empty constants
• wrapWithPythonDecoderExpr directly
• concatChunks cross-boundary text merging

Add PyBuilder to the root aggregate, and add a unit spec covering the gaps above.

Task Type

• Refactor / Cleanup
• Testing / QA