Fix default port handling in Target class to prevent invalid URLs

[coderabbitai]

Problem

The Target.getPort() method currently returns the raw field value (default -1), which causes invalid URLs when concatenated directly in methods like ShadowingInterceptor.buildTargetUrl(). This produces URLs like http://example.com:-1/path.

Proposed Solution

Modify getPort() to return sensible defaults:

• Return 443 when sslParser != null (HTTPS)
• Return 80 otherwise (HTTP)
• Return the stored port value when it's explicitly set (> 0)

This ensures valid URLs while maintaining backward compatibility for code that checks if the port is unset.

Context

• File: core/src/main/java/com/predic8/membrane/core/proxies/Target.java
• Related PR: JsonSchemaGenerator: Refinements for YAML completion #2240
• Review comment: JsonSchemaGenerator: Refinements for YAML completion #2240 (comment)
• Reported by: @predic8