fix(core): handle tool execution errors with isError flag and add tests

[fabiooliveir]

Description

This PR addresses two key improvements for the Hostinger MCP Server:

1. MCP Specification Compliance (isError: true):

   • Previously, HTTP errors (4xx/5xx) and unhandled execution exceptions either returned raw response payloads as success or threw unhandled JSON-RPC exceptions.
   • Now, tool execution errors return a standard MCP CallToolResult with isError: true and the error details in the text content, allowing LLM clients (Claude, Cursor, Gemini, Copilot) to understand and self-correct on API failures.
   • Protocol errors for missing tools (Tool not found) are preserved.

2. Automated Test Suite & Hygiene:

   • Added automated tests in test/runtime-errors.test.js using Node.js native test runner (npm test) covering success (2xx), API errors (400, 401, 422, 5xx), connection drops (ECONNREFUSED), and custom tool validations.
   • Added a root .gitignore to prevent committing dist/, node_modules/, and sensitive .env credentials.

Verification

• npm test: 8/8 tests passing.
• npm run build: TypeScript compilation succeeded without errors.