ext/reflection: Add ReflectionConstant::inNamespace() method

[khaledalam]

Description

Fix #19557

This PR adds the missing inNamespace() method to ReflectionConstant to achieve full parity with ReflectionClass and ReflectionFunctionAbstract for API consistency.

Background

ReflectionConstant already has:

• getShortName() - returns the short name without namespace
• getNamespaceName() - returns the namespace name

But was missing:

• inNamespace() - returns boolean indicating if in a namespace

[iluuu1994]

LGTM, thanks @khaledalam! There were some recent policy changes:

https://github.com/php/policies/blob/main/release-process.rst#feature-selection-and-development

A core developer MAY also request that the feature be discussed on the internals mailing list, in which case an additional two-week period MUST pass without objection or RFC request before the feature can be merged.

Trivial new functions should briefly be discussed on the internals mailing list. If nobody objects after two weeks, this should be ok to merge.

[iluuu1994]

Correction, I missed this part:

Internal API changes (those that do not affect the user-facing API), as well as user-facing features in extensions and SAPIs, do not require an RFC unless a core developer (someone with commit access to php-src) raises an objection or requests an RFC within one month of the implementation pull request being opened.

So I suppose no discussion is needed. (I don't object to this function). So @DanielEScherzer, please merge if you see appropriate.