Fix #406: Prevent multiple server sessions at startup

[isc-klu]

Problem

When VS Code opens with many tabs, each tab concurrently calls makeRESTRequest(), creating multiple sessions to the same server.

Solution

Serialize REST requests per server using promise-based queueing:

• First request authenticates and stores session cookie
• Concurrent requests wait and reuse the session cookie
• Subsequent requests use cached cookies immediately

Changes

• Changed cookiesCache to store promises instead of raw cookie arrays
• Made getCookies()/updateCookies() async with queue logic
• Added updateCookies() in catch block to ensure promises always resolve