Remove encapsedStringPart.nonString baseline and fix its issue

[dpanta94]

Removes in total 1 errors.

Removes errors from the below phpstan baselines and fixes the issues that they were covering:

• encapsedStringPart.nonString.neon

How ?

Better specify the shape of the return type.

Trac ticket: https://core.trac.wordpress.org/ticket/65817

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Opus 5
Used for: Help with suggesting how to fix the phpstan output for the specific errors we are removing the baselines for. The actual result has been reviewed and are owned by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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