P1-M4-T1: Shared HTTP client utility

[Rihoj]

Task ID: P1-M4-T1

Implement a shared HTTP client with sensible timeouts and retry/backoff (exponential jitter).

Acceptance criteria:

• Utilities available to tools
• Unit tests for retry/backoff using a test server

Estimate: 1d
Labels:["infra","networking","phase:1","milestone:P1-M4"]

Dependencies:

• P1-M1-T2: Server initialization & config #2

Project: https://github.com/orgs/rayprogramming/projects/2