fix: serve external $ref fixtures from a local HTTP server in tests

[rafael-goterris]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: N/A

Several external $ref tests resolve their references against https://raw.githubusercontent.com/apiaddicts/sonaropenapi-rules/.... This forces the test suite to make outbound HTTPS calls to GitHub during mvn test, which fails in restricted or offline environments (corporate networks, CI runners without egress, air-gapped setups) and also makes the tests dependent on the availability of GitHub and on the contents of the master branch at test time.

Affected tests: OAR031 (v2 and v3), OAR094, OAR068, OAR086.

What is the new behavior?

• Added a new test utility ExternalRefHttpServer that starts a lightweight in-process HTTP server on http://localhost:18089, serving the YAML files under src/test/resources/externalRef/.
• BaseCheckTest.beforeClass starts the server once for the test suite, so all checks resolving external $refs do it against localhost` instead of the public internet.
• Updated the affected fixtures (OAR031 v2/v3, OAR094, OAR068, OAR086) to point to http://localhost:18089/<file>.yaml instead of the raw.githubusercontent.com URLs.
• Bumped version to 1.3.6 in pom.xml and added the corresponding entry to CHANGELOG.md.

Other information

• The fixture files under src/test/resources/externalRef/ are unchanged; only the URLs that reference them have been switched to localhost.
• The HTTP server binds to 127.0.0.1 only and is shut down via a JVM shutdown hook, so it does not leak between builds.
• No production code is modified — changes are scoped to the test suite, the version bump and the changelog.