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/nightly.yml

@@ -338,7 +338,7 @@ jobs:
             --url http://localhost:13490 \
             --checks not_a_server_error \
             --include-path-regex "^/api/v1/(tools|health|info)" \
-            --exclude-path-regex "/(remove-background|upscale|blur-faces|erase-object|ocr|ocr-pdf|colorize|enhance-faces|noise-removal|smart-crop|red-eye-removal|restore-photo|passport-photo|transparency-fixer|ai-canvas-expand|transcribe-audio|auto-subtitles|background-replace|blur-background)(/[a-z-]+)?$" \
+            --exclude-path-regex "/(remove-background|upscale|html-to-image|blur-faces|erase-object|ocr|ocr-pdf|colorize|enhance-faces|noise-removal|smart-crop|red-eye-removal|restore-photo|passport-photo|transparency-fixer|ai-canvas-expand|transcribe-audio|auto-subtitles|background-replace|blur-background)(/[a-z-]+)?$" \
             --max-examples 25 \
             --report junit \
             --report-dir st-report