chore: bump version to 2.8.3 and update changelog

- Updated version in package.json and package.runtime.json
- Updated version badge in README.md
- Added comprehensive changelog entry for v2.8.3
- Fixed TypeScript lint errors in test files by making env vars optional
- Fixed edge-cases test to include required NODE_ENV

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.8.2",
+  "version": "2.8.3",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "bin": {