feat: Extension Host Stabilization & Marketplace Support

[meftunca]

This PR addresses critical stability issues in the SideX extension host and enhances compatibility with marketplace extensions.

Key Changes

1. Extension Host Stabilization

• Platform Readiness Gate: Implemented a synchronization mechanism in host.cjs to defer extension activations until the backend environment and document state are fully synchronized.
• Shim Hardening: Added defensive null/undefined checks for vscode.window.activeTextEditor and visibleTextEditors.
• DiagnosticCollection Binding: Explicitly bound methods in DiagnosticCollection to prevent this context loss.

2. Marketplace & Nightly Extension Support

• Engine Compatibility: Bypassed strict engine version and API proposal checks when SIDEX_TAURI is detected.
• CodeLens RPC Infrastructure: Implemented a full RPC bridge for CodeLens resolution.

3. Core Infrastructure & Build

• Vite 8 & Rolldown Migration: Upgraded build pipeline to Vite 8/Rolldown.
• macOS Environment Synchronization: Fixed PATH inheritance for macOS.
• Remote Agent Compatibility: Added stubbed handlers for remote_internal_set_active_folders.