[console-browser-keepalive-unknown-error] clarify keep-alive WebSocket reset logging

[atomantic]

Summary

• suppress routine keep-alive WebSocket errors during browser shutdown
• report the most useful available reset detail and warn that reconnect is scheduled

Validation

• node --check browser/server.js
• git diff --check
• npm test --prefix server — 2,165 test files passed, 1 skipped; 43,403 tests passed, 36 skipped; one pre-existing import-budget guard failed (103,592 vs 103,240)
• local Claude review (medium effort): NO FINDINGS

Closes #7065

[atomantic]

CI is blocked by a pre-existing server import-budget failure, not by this PR. Run 34665187373 reports server/lib/importScoping.test.js: 103,592 static module instantiations exceeds the 103,240 budget; the PR diff only changes browser/server.js and adds no imports. I reproduced the identical failure on the exact origin/main base 2e65263. The CI failure short-circuited the sibling jobs, so this PR remains open with its claim markers for the baseline budget repair; no merge or protection bypass was attempted.

[atomantic]

Review follow-up: Claude ran once in enforced read-only plan mode (Read/Glob/Grep only, safe/restricted mode, no hooks, plugins, MCP, network or write tools). Exit 0; tracked/index/untracked baseline hashes were preserved. Status: no-verdict / optional inconclusive (1/1 rounds) because the output included prose outside the strict finding-block contract.

I checked both reported concerns:

• The claimed perpetual superseded-socket reconnect loop is not reachable through the current callers: startup calls once, and the close handler clears the current socket before scheduling the reconnect. The retry timer is also cleared before retry. This PR does not change that lifecycle.
• The claimed error-only failed handshake was disproved with a local HTTP-400 WebSocket handshake on Node v24.14.1: events were error then close; the close handler schedules the reconnect. The installed Undici implementation likewise dispatches error and then close in its connection-closed handler.

No review fixes were warranted. Per the explicitly optional one-round reviewer configuration, this inconclusive pass does not block merge.

Merged current main into this branch to incorporate the already-landed import-budget repair. Validation: all 68 import-scoping tests pass; node --check browser/server.js and git diff --check pass. The final PR diff remains only the original four-line logging change. Waiting for current-head CI before merging.