[JuliaSyntax] Fix `sourcetext` for `SyntaxTree` with `LineNumberNode` provenance (#61267)
When a `SyntaxTree` node has `LineNumberNode` as its source provenance
(which happens for nodes created via `expr_to_est` during old-style
macro expansion), the generic `sourcetext(x)` would call
`view(sourcefile(x), byte_range(x))` which fails with `MethodError`
because `view(::LineNumberNode, ::UnitRange)` is not defined.
Add a specialized `sourcetext(::SyntaxTree)` method that checks for
`LineNumberNode` and returns `SubString("")`, consistent with the
existing `sourcetext(::LineNumberNode)` method.
Fixes aviatesk/JETLS.jl#583
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>