fix(sse): classify missing Chromium as a Z.ai host/config cooldown (#13232)

[diegosouzapw]

Closes #13232

Root cause (short)

The Z.ai web transport (open-sse/executors/zai-web.ts::fetchThroughBrowser) drives a real
headed Chromium browser via Playwright to get past Z.ai's CAPTCHA. When the local Playwright
Chromium binary is missing, chromium.launch() throws browserType.launch: Executable doesn't exist at .... The fetchThroughBrowser catch block had no classification for that failure and
always wrapped it as a plain 502 with no extra headers — a status that trips the
whole-provider circuit breaker (AGENTS.md → "Provider Circuit Breaker": 408/500/502/503/504
all trip it) as if the upstream itself were failing, instead of applying a short host/config
cooldown.

This is the exact same failure class already handled for GeminiWebExecutor in issue #3516:
isMissingBrowserExecutable() detects the message shape and the executor returns 503 +
X-Omni-Fallback-Hint: connection_cooldown, which accountFallback (via
serviceSupervisorCooldown() in open-sse/config/errorConfig.ts) treats as a short,
non-exponential connection cooldown and explicitly skips the provider breaker. zai-web.ts never
got that same treatment.

Fix

• Extracted isMissingBrowserExecutable() out of open-sse/executors/gemini-web.ts into a new
  shared helper open-sse/executors/browserExecutableCheck.ts, re-exported from gemini-web.ts
  for backward compatibility (an existing test imports it from there).
• open-sse/executors/zai-web.ts::fetchThroughBrowser's catch block now checks
  isMissingBrowserExecutable(rawMessage) before falling back to the generic 502; on a match it
  returns 503 + X-Omni-Fallback-Hint: connection_cooldown with an actionable message ("Z.ai
  requires the Playwright Chromium browser, which is not installed. Run npx playwright install chromium on the host...").
• open-sse/utils/error.ts::makeExecutorErrorResult gained an optional 5th
  extraResponseHeaders parameter (merged into the Response's headers) so zai-web.ts can set
  the fallback hint without constructing the Response by hand — every other existing caller
  (14 executors) is unaffected since the parameter is optional.

This does not touch describeZaiBrowserFailure() (zai-web.ts:440-449), which handles a
different case — a non-2xx status returned by browserBackedChat() mid-flow, not a thrown
launch error.

Per the plan-file's own scope note, the equivalent gap in claude-web, duckduckgo-web.ts, and
cloudflare-playground.ts was flagged as optional/stretch and is not covered by this PR to
keep the diff scoped to the confirmed zai-web bug.

Regression test

tests/unit/zai-web-missing-browser-executable-13232.test.ts — forces chromium.launch() to
fail with the exact reporter error by pointing PLAYWRIGHT_BROWSERS_PATH at an empty temp
directory, then asserts the classified 503 + hint response.

RED (on unfixed zai-web.ts, captured by temporarily restoring the pre-fix file):

✖ returns a classified 503 + X-Omni-Fallback-Hint: connection_cooldown instead of a bare 502 (contrast: gemini-web.ts isMissingBrowserExecutable, #3516)
  AssertionError [ERR_ASSERTION]: zai-web must classify a missing local Chromium install as a host/config error (503), not a generic retryable 502 that trips the whole-provider circuit breaker.
  502 !== 503
ℹ tests 1
ℹ pass 0
ℹ fail 1

GREEN (fixed code):

✔ returns a classified 503 + X-Omni-Fallback-Hint: connection_cooldown instead of a bare 502 (contrast: gemini-web.ts isMissingBrowserExecutable, #3516) (6729.395077ms)
ℹ tests 1
ℹ pass 1
ℹ fail 0

Gates run

• npx eslint --suppressions-location config/quality/eslint-suppressions.json <changed files> → clean, no new warnings.
• npm run check:open-sse-typecheck → openSseTypecheckErrors=0, OK.
• node scripts/check/check-file-size.mjs → no offenders among touched files (one pre-existing, unrelated open-sse/utils/stream.ts frozen-file offender not touched by this PR).
• node scripts/check/check-complexity.mjs → OK, 2824 violations vs baseline 3218 (no new offender in touched files).
• node scripts/check/check-cognitive-complexity.mjs → OK, 1276 violations vs baseline 1437 (no new offender in touched files).
• node scripts/check/check-test-discovery.mjs → OK, new test file discovered, no new orphans.
• DATA_DIR=$(mktemp -d) node --import tsx/esm --test --test-force-exit tests/unit/gemini-web-missing-browser-3516.test.ts tests/unit/executor-zai-web.test.ts tests/unit/zai-web-attachment-mime-contract.test.ts tests/unit/zai-web-auth-semantics.test.ts tests/unit/zai-web-silent-empty-repro.test.ts → 45/45 pass (confirms the gemini-web.ts extraction is behavior-preserving and the existing zai-web suites are unaffected).

Existing tests aligned

None needed alignment — no pre-existing assertion encoded the old buggy 502 contract.

Note: tests/unit/zai-web-stream-error-boundary.test.ts timed out locally
(spawnSync ... ETIMEDOUT, a hardcoded 60s child-process timeout) on this heavily-loaded
devbox. It spawns an unrelated process-isolated fixture that never touches
fetchThroughBrowser/browserBackedChat — confirmed unrelated to this change and reproduces
the same way without any of this PR's edits applied.