Fix Maven plugin content-type for YAML AsyncAPI files (#6712)

[EricWittmann]

Summary

• Fixed Maven plugin to correctly set content-type to application/x-yaml for YAML files during artifact registration
• Added test coverage to verify content-type is preserved for both YAML AsyncAPI files and JSON Avro schemas
• Added integration test for AsyncAPI 3.0 with Avro references to reproduce the user's scenario

Root Cause

When using the Maven plugin with autoRefs enabled, the plugin determined the correct content-type from the file extension but failed to set it on the artifact object before registration. This caused the registration code to default to application/json, resulting in YAML content being registered with the wrong content-type.

Changes Made

Maven Plugin Fix (RegisterRegistryMojo.java:177-180):

• After determining content-type from file extension, now sets it on the artifact object if not explicitly provided by user
• Ensures YAML files (.yml, .yaml) are registered with application/x-yaml
• Preserves user-specified content-types (doesn't override explicit values)

Test Coverage (RegisterAsyncApiAvroAutoRefsTest.java):

• Enhanced test to capture and verify content-type headers sent during registration
• Validates AsyncAPI YAML files are registered with application/x-yaml
• Validates Avro schema files are registered with application/json

Integration Test (GroupsResourceTest.java):

• Added test case using AsyncAPI 3.0.0 with Avro references (matching user's setup)
• Verifies YAML content is preserved and parseable with ?references=REWRITE
• Includes test resources: asyncapi-shopping-cart.yml and avro/ShoppingCartCreated.avsc

Related Issue

Fixes #6712

Test Plan

• Run RegisterAsyncApiAvroAutoRefsTest to verify Maven plugin sets correct content-types
• Run GroupsResourceTest#testGetArtifactVersionWithReferencesYamlContentType to verify YAML handling
• Test with AsyncAPI 3.0.0 YAML files containing Avro references
• Verify YAML files are registered as application/x-yaml not application/json