fix: Bump Nokogiri to 1.19.4 (#14807)

# Pull Request Template

## Description

This updates Nokogiri from `1.19.3` to `1.19.4` so the bundle-audit lint
step stops flagging the newly published Nokogiri advisories. Chatwoot's
direct Nokogiri usage appears limited to ordinary HTML/XML parsing and
selector traversal, but the locked dependency is below the patched
floor, so the safe remediation is the patch-level upgrade rather than an
advisory override.

Fixes
[CW-7397](https://linear.app/chatwoot/issue/CW-7397/upgrade-nokogiri-to-1194-for-bundle-audit-advisories)

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

Reference failed build in CI because of bundle audit:
https://app.circleci.com/pipelines/github/chatwoot/chatwoot/114392/workflows/8a252bf9-5e58-45fd-af18-a32dbebe978b/jobs/160423

- `bundle exec bundle audit update && bundle exec bundle audit check -v`
passed with no vulnerabilities found.
- `bundle exec rspec spec/services/website_branding_service_spec.rb
spec/presenters/html_parser_spec.rb
spec/enterprise/services/enterprise/website_branding_service_spec.rb
spec/enterprise/services/captain/tools/simple_page_crawl_service_spec.rb`
passed with 27 examples and 0 failures.
- `bundle exec rubocop app/services/website_branding_service.rb
app/presenters/html_parser.rb
enterprise/app/services/page_crawler_service.rb
enterprise/app/services/captain/tools/html_page_parser.rb
enterprise/app/services/captain/tools/simple_page_crawl_service.rb`
passed with no offenses.
- `git diff --check` passed.

Note: broad `bundle exec rubocop --parallel` still reports existing
generated DB/schema offenses unrelated to this lockfile-only dependency
bump.

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

Gemfile.lock

@@ -598,14 +598,14 @@ GEM
     newrelic_rpm (9.6.0)
       base64
     nio4r (2.7.5)
-    nokogiri (1.19.3)
+    nokogiri (1.19.4)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
-    nokogiri (1.19.3-arm64-darwin)
+    nokogiri (1.19.4-arm64-darwin)
       racc (~> 1.4)
-    nokogiri (1.19.3-x86_64-darwin)
+    nokogiri (1.19.4-x86_64-darwin)
       racc (~> 1.4)
-    nokogiri (1.19.3-x86_64-linux-gnu)
+    nokogiri (1.19.4-x86_64-linux-gnu)
       racc (~> 1.4)
     oauth (1.1.6)
       auth-sanitizer (~> 0.2, >= 0.2.1)