Implement upstream reference validation in API configuration

Context

During the review of PR #403, it was identified that the validateUpstreamRef function in gateway/gateway-controller/pkg/config/api_validator.go (line 203) is currently a stub with a TODO comment.

The Ref field is already part of the documented Upstream schema (intended to reference a predefined upstreamDefinition), and the validation function is actively invoked when up.Ref is set. However, it currently returns no validation errors, leaving this feature without proper validation.

What needs to be done

Implement the validation logic for upstream references in the validateUpstreamRef function to ensure that:

• Referenced upstream definitions exist and are valid
• The reference format follows expected patterns
• Appropriate error messages are returned for invalid references

Related

• Pull Request: https://github.com/wso2/api-platform/pull/403
• Review Comment: https://github.com/wso2/api-platform/pull/403#discussion_r2612848203
• Requested by: @nimsara66

This feature was noted in the PR objectives as "Advanced upstream configuration referencing capability" and is part of the enhanced API configuration support.