Add Java contract support to JuLC Playground

[satran004]

Summary

• Add full Java validator support to the playground: compile, test, and evaluate Java smart contracts directly in the browser
• Disable JRL language support (Java-only for now)
• Add production hardening: input validation, error sanitization, rate limiter security, configurable CORS, request logging
• Add implicit stdlib imports so users can reference ContextsLib, Builtins, etc. without explicit import statements
• Rename JrlPlaygroundServer → JulcPlaygroundServer

What's New

Java Compilation Pipeline

• Full Java → PIR → UPLC → Blueprint compilation with stdlib auto-discovery
• Java validator test execution with @param, datum, redeemer encoding and ScriptContext building
• Expression evaluation endpoint (/api/eval) for quick REPL-style eval
• Three bundled Java examples: SimpleSpending, VestingValidator, MintingPolicy

Editor Intelligence

• Java autocomplete with type-aware instance methods and library method discovery
• Method parameter hints (SignatureHelpProvider) — shows param names/types as you type
• Hover docs for stdlib and Builtins methods
• Go-to-definition for user-defined records and sealed interfaces
• Dual-tab editor (Validator / Library) with library source integration

Production Hardening

• InputValidator with max-length checks (100KB source, 100KB library, 10KB expression)
• Error message sanitization — no stack traces or internal details leaked to client
• Rate limiter only trusts X-Forwarded-For when JULC_BEHIND_PROXY=true
• Configurable CORS via JULC_CORS_ORIGINS env var
• Path traversal guard for native image static file serving
• Request logging middleware with per-endpoint duration tracking
• Enhanced /api/health with sandbox slot availability
• AbortController for frontend auto-check to prevent race conditions
• Frontend error states for failed example/scenario loading
• Proper 408/429 feedback in UI

Compiler: Implicit Stdlib Imports

• ImportResolver now adds com.bloxbean.cardano.julc.stdlib and com.bloxbean.cardano.julc.stdlib.lib as implicit wildcard packages (like java.lang.*)
• Only resolves types registered in knownFqcns (14 stdlib classes) — no risk of unintended captures
• Replaces the earlier suffix-matching approach that broke existing tests