Add Connection: keep-alive to SSE default headers

[Copilot]

SSE fetch requests were missing Connection: keep-alive, which is required for persistent streaming connections to work reliably across proxies and load balancers.

Changes

• dmax.js: Added H_CONNECTION = 'connection' constant; included connection: keep-alive in ACT_HS_SSE alongside existing accept, cache-control, and pragma headers

// Before
const ACT_HS_SSE = Object.freeze({ [H_ACCEPT]: 'text/event-stream', [H_CACHE_CONTROL]: 'no-cache', [H_PRAGMA]: 'no-cache' })

// After
const ACT_HS_SSE = Object.freeze({ [H_ACCEPT]: 'text/event-stream', [H_CACHE_CONTROL]: 'no-cache', [H_CONNECTION]: 'keep-alive', [H_PRAGMA]: 'no-cache' })

• tests/dmax.size.limits.json: Updated byte limit to reflect the addtion

• Fixes Ensure we are using Connection: keep-alive for sse by default #146