fix(nightly): de-flake matrix timeouts, exclude browser tool from fuzz (#349)

Systematic round - attack the failure classes, not one bug at a time:

- Flakiness (Coverage/Extended Matrix flipped green<->red): the generated
  format-matrix conversion tests hardcode a 30s/60s per-test timeout that the
  jobs' VITEST_TEST_TIMEOUT can't override. Heavy conversions under coverage
  instrumentation / full matrix intermittently exceed it. Raise to 120s.
- Schemathesis: html-to-image returns 503 when its headless browser isn't
  present (always, in the fuzz env) - same expected-unavailable class as the
  AI 501s. Exclude it from the fuzz (it can't be exercised without a browser).
- Baseline regen: bump timeout 60->120 min so it can finish now that the
  route sweep (#347) lets the visual specs pass instead of failing+retrying.

.github/workflows/update-visual-baselines.yml

@@ -11,7 +11,7 @@ jobs:
   update-baselines:
     name: Regenerate linux visual baselines
     runs-on: ubuntu-latest
-    timeout-minutes: 60
+    timeout-minutes: 120
     # The e2e webServer (playwright.config.ts) creates an isolated database via
     # tests/e2e-pg-create-db.cjs, so it needs Postgres + Redis on localhost.
     # Without these services the webServer fails with "failed to create database".