Fix B4: PropertyProxy attribute inheritance via merge getAttributes()

[torian257x]

Summary

PropertyProxy now merges its local attributes with the underlying property's attributes via getAttributes() instead of enumerating attribute types in processReference(). When the same FQCN appears in both lists, the proxy's local version wins (SchemaName, JsonPointer). All other attributes (Required, ReadOnly, WriteOnly, Deprecated) are inherited from the underlying.

What the maintainer asked

"B4 might be the correct solution but it misses some attributes (e.g. ReadOnly, WriteOnly, Deprecated). Probably the mechanic would be to merge all attributes which are not explicitly set on the proxy from the underlying property on read to ensure all known attributes are present"

Response: The previous approach enumerated attribute types for explicit overwrite on the proxy. This missed ReadOnly, WriteOnly, and Deprecated. The new PropertyProxy::getAttributes() merge covers ALL attribute types generically. PostProcessors that add attributes to the underlying property (now or in the future) will automatically appear on the proxy. Local wins: SchemaName and JsonPointer are set per-instance. Everything else inherits.

Why this is correct

• Required correctness: resolveReference() includes $required in its cache key. Different requiredness → different keys → separate Properties, never a proxy. Inherited Required is always correct.
• No recursion: getAttributes() calls the underlying Property which is never a Proxy.
• Null safety: Proxies resolve before property processing completes; getAttributes() is only called during rendering (long after).

Changes

• PropertyProxy::getAttributes() — new merge method
• PropertyFactory::processReference() — only set SchemaName on PropertyProxy; all other attributes inherit
• AttributesTrait — $phpAttributes visibility private→protected

Test

B4PropertyProxyAttributeTest.php

[torian257x]

This PR addresses the B4 feedback from PR #150. I am an AI assistant working on behalf of @torian257x.

"B4 might be the correct solution but it misses some attributes (e.g. ReadOnly, WriteOnly, Deprecated). Probably the mechanic would be to merge all attributes which are not explicitly set on the proxy from the underlying property on read."

Approach: PropertyProxy::getAttributes() now merges local proxy attributes with the underlying property's attributes via a generic merge. When the same attribute FQCN appears in both lists, the proxy's local version wins. This covers ReadOnly, WriteOnly, Deprecated and any future attribute types automatically.

The old approach (enumerating SchemaName, JsonPointer, Required in processReference()) was removed. SchemaName is still set per-instance on the proxy. Everything else inherits.

Why correct: resolveReference() includes $required in its cache key, so different requiredness produces separate Properties (never a proxy). Inherited Required is always correct.