fix(extension): omit credentials from daemon ping (fixes #2278)

[kagura-agent]

Problem

Fixes #2278. The extension can silently stop before its WebSocket connection when the daemon /ping returns HTTP 431.

A large localhost cookie jar can exceed the Node default HTTP header limit. Because the ping used the browser default credential mode, Chrome attached those cookies, the daemon answered 431, and the extension quietly scheduled another retry without exposing the response.

Fix

• send only the extension /ping request with credentials: omit, so browser cookies are not attached
• log non-OK ping status codes before scheduling the normal reconnect
• keep connection errors silent because a stopped daemon is the expected idle state

This deliberately does not raise the daemon header ceiling globally. The reproduced failure is the HTTP ping, and omitting credentials resolves that narrow path without expanding parsing limits for every HTTP endpoint or WebSocket upgrade.

Verification

Exact head 62d1f202 rebased onto main 50565efd:

• npx vitest run --project extension — 3 files, 97 tests passed
• npm run typecheck — passed
• cd extension && npm run build — passed
• git diff --check origin/main...HEAD — passed

The final diff contains only extension/src/background.ts and extension/src/background.test.ts.

[kagura-agent]

Hi @jackwener 👋 — gentle ping. This fix for #2278 (silent 431 wedge from localhost cookie overflow) has been open for 3 days with all CI green. Would appreciate a review when you have a moment. Happy to adjust anything.