Go: Fix multiple bugs in rewrite-go module

[knutwannheden]

Motivation

Code review of the rewrite-go module identified several bugs and missing visitor coverage that would impact correctness and robustness of the Go language support.

Summary

• Fix panic recovery in RPC server — safeHandleRequest now returns a proper JSON-RPC error response instead of nil when a panic is caught, preventing silent session death
• Fix parser to handle multiple import blocks — Go allows import "fmt" followed by import "os" (common in generated code). The parser previously only processed the first import block, corrupting cursor position tracking for the rest of the file. A new ImportBlock marker tracks block boundaries, with corresponding printer, RPC serialization, and Java marker support.
• Fix visitAndCast/visitExpression nil panics — These helpers now handle nil returns from visitors instead of panicking on the type assertion
• Fix VisitCompilationUnit to visit PackageDecl and Imports — These were previously skipped entirely, meaning no recipe visitor could transform package names or imports
• Remove dead code in mapArrayType — Unreachable closePrefix computation that was immediately overwritten

Test plan

• Added TestParseMultipleImportBlocks — parse-print round-trip for import "fmt" / import "os"
• Added TestParseMultipleGroupedImportBlocks — parse-print round-trip for multiple import (...) blocks
• Added TestVisitorReturningNilDoesNotPanic — visitor returning nil for a Return node doesn't crash
• Added TestVisitorVisitsImports — confirms import nodes are visited (was 0, now 2)
• Added TestVisitorVisitsPackageDecl — confirms package decl identifier is visited
• All existing Go tests pass
• Java compilation passes