Scala parser: wrap non-Expression J trees in S.StatementExpression at visitUnknown fallbacks

[jkschneider]

Summary

• Mirrors the then branch's expression-wrapping fallback in visitIf's else branch so if (x) a else b with expression operands (e.g. if (flag) "yes" else "no") parses without throwing UnsupportedOperationException.
• Adds case j: J => new S.StatementExpression(Tree.randomId(), j) fallbacks at ~25 visitUnknown sites so Scala idioms where an expression appears in an expression-required slot (but maps to a non-Expression J) no longer crash the parser. Affected visitors include: visitWhileDo, visitForDo, visitReturn, visitThrow, visitMatchImpl, visitAssign, visitInfixOp / visitBinaryOperation / visitInfixMethodCall, visitPrefixOp / visitPostfixOp, visitParentheses, visitSelect, visitTyped, visitTypeApply (asInstanceOf/isInstanceOf), visitAppliedTypeTree, method-call target, ArrayAccess, NewArray/NewArrayWithType, NewClassWithArgs, annotation args, tuple elements. Try body/finalizer also now accept Statement (matching existing ParsedTry handling).
• Fixes ScalaParser.sourcePathFromSourceText — was hard-coded to "file.scala", causing multi-file tests to collide on a single path and garble each other on print round-trip. Extracts package + class name via a shared derivedRelativePath helper (logic reused from parse()).

Motivation

Integration-testing prethink quality recipes against Scala sources surfaced two concrete blockers:

1. UnsupportedOperationException: Unmapped Scala AST node: If at [116..141] source=if (flag) "yes" else "no" — the elsep visit in visitIf was missing the expression-wrapping fallback that thenp already had.
2. Two-file test case with distinct packages collided on src/main/scala/file.scala, producing garbled print output.

The else fix exposed that the same defensive pattern was missing at many other sites. The fallback is mechanical: if visitTree returns a J that isn't exactly an Expression (e.g. a Statement like J.If used as a value in Scala), wrap it via S.StatementExpression (which implements both Expression and Statement). This keeps existing behavior for the happy path and replaces a crash with a round-trippable LST in edge cases.

Test plan

• 7 new regression tests in ControlFlowTest and CompilationUnitTest:
  • ifElseExpressionLiteralOperands, ifElseExpressionIdentifierOperands
  • returnWithIfExpression, assignmentWithIfExpression
  • binaryOpWithIfExpressionOperand, tupleWithIfExpressionElements, throwWithIfExpression
  • whileWithUnitBody
  • multipleFilesGetDistinctPaths
• Full rewrite-scala test suite (~400 tests across 30+ classes) passes with 0 regressions.