Coordinate proxy decompilation and harden JDTLS response routing

[tartarughina]

• Move jdt:// resolution to a bounded coordinator with request deduplication, priorities, deadlines, cancellation, and negative caching.
• Add session-isolated, atomically written decompiled-source caches with valid filenames and encoded cross-platform file URIs.
• Serialize proxy output and safely route proxy-owned, late, suppressed, and rewritten responses.
• Handle workspace-symbol supersession and proxy shutdown without blocking transport threads.
• Improve completion snippet sanitization and refactor stdin/stdout routing into testable handlers.