fix(core): unref the CDP deadline so it can't delay process exit

[rishigupta1599]

Summary

Follow-up to the CDP send deadline. The deadline is a watchdog, but its timer was not unref'd, so it holds the Node event loop open.

An orphaned send — one whose reply is never coming — leaves that timer live for the full deadline. This is not exotic: it happens routinely when a page closes with a Fetch.continueResponse still in flight. By then the real work is done, so the CLI sits idle for up to two minutes after printing Finalized build #… before exiting.

I hit this intermittently (once in four runs) while capturing a real Storybook: the run went quiet after finalizing and exited 116s later, exactly when the deadline fired.

[percy:core] Finalized build #10544: https://percy.io/…
[percy:client] Sending Build Events
[percy:core:session] Protocol timeout (Fetch.continueResponse) after 120000ms (116197ms)   <- 116s of idling

A watchdog should never be the reason a process cannot exit. unref() matches the existing precedent in server.js, and the timer still fires normally whenever anything else is keeping the loop alive — which is the case for every send that matters.

Test plan

• Two new specs (one per send path) capture the created timer and assert hasRef() === false, so a future change that re-refs the watchdog fails the suite.

• All 13 specs in packages/core/test/unit/session.test.js pass; eslint clean.

• Verified in a real process with a probe holding one orphaned send at a 30s deadline:

  	process exit
  unref'd (this PR)	346ms
  ref'd (before)	30162ms

Notes

• Deliberately unguarded .unref() rather than .unref?.(): server.js:226 already does the same, and an optional call would add a branch whose false arm is unreachable in Node, breaking the 100% branch-coverage gate.
• The new spec helper swaps global.setTimeout to capture the deadline, and hands the real setTimeout to the callback so its own waits aren't captured — otherwise there's no telling which timer is which.

🤖 Generated with Claude Code